Most thorny shrubs are just thorny, no more. Try poison oak.

The very first plant that comes to mind is one of the Japanese Hardy Oranges, Poncirus trifoliata 'Flying Dragon'. Of all of the cultivars, this one will probably be easiest to find because it's so interesting AND will not take over your entire yard. It's a VERY cool plant.
Perfectly hardy in your area!

Wintergreen barberry would be a possible evergreen species to try....should be readily available....such a pretty plant.

I'd certainly suggest motion detector lights and cameras, too. Perhaps save your pennies for a tall, privacy fence built on the inside of 'his' fence.

Deshima, my sympathies. I also have a cave man neighbor (my apologies to cave men, but you get the idea). I don't have any specific suggestions for your zone, but isn't there some kind of plant that stinks? Of course he might not notice that. What about bamboo which is very thick and invasive, though not flesh eating ;-).

My neighbor-from-he** happens to have no criminal history - he's in Law Enforcement!
Elaine

You could try Colletia paradoxa - it is seriously vicious despite looking kind of strokeable.

opuntias (prickly pear) would be a physical deterrent and some folk are irritated by the smaller "hairs" on the pads.
pyracantha is a very thorny shrub with attractive berries and can be a quick grower. i was thinking that the common name ("firethorn") indicated a burning skin reaction to the thorns(?) but not sure.
the thorniest rose i have ever grown is 'sir thomas lipton', with white flowers and long, strong canes armed heavily with thorns.
all things considered, i would also opt for my own fence just inside my property line. if he knows you enjoy gardening, he may decide to apply herbicide to anything you plant along the border.

Thanks for all of the advise I am going to be studying your suggestion and will let you know what I decide, many thanks and any other suggestions are most appreciated.

My sympathies to you! I know how you feel, we live in the boonies and all our neighbors are jerks. The one *&$@$@! pointed a shotgun at my husband for NO reason, and I mean NO reason (they are trash and nuts too), and nothing ever got done about it. The judges around here don't care either. The cops got mad at US for making them come out there! The law doesn't seem to care about the violent ones, just the druggies.

As to what to plant, if you don't mind a spreading noxious weed, try the 'devil's turnip'. Sounds perfect for your evil neighbor. The sap from the plant will irritate the skin, it can grow into huge proportions and spreads like crazy. But it IS a noxious weed, so beware of that.

Read the bottom of the page to get 2 accounts of the plant also called 'white byron'. http://davesgarden.com/pf/go/31800

You could also try growing (sneak it on his property at night, plant it at night, especially if he's got a swampy area) stinging nettle. That stuff comes back from its roots every year. It loves damp soil.

bougainvillea have bad thorns and look really pretty!

Catbriar can become thorny masses pretty quickly. They are tough and wiry.

Dogwood rose is considered a noxious weed but it will grow really large and become a huge bush with really bad thorns. It smells nice when blooming though.

You could always plant bamboo and let it run! It might even take over his yard.

My sympathies, I know how you feel. Our neighbor is a coward, but acts real tough when he's far enough away. He tried to run my husband off the road while my husband was on his motorcycle (he was such a wimp he didn't try to do it too blatantly, but he only gave him like 2 feet of road). My husband got him back though, the jerk wrecked his truck, and we saw the insurance truck go over to his trailer. My husband waited at our gate until the guy drove by (our loser neighbor can't see our front gate from his place). He told the insurance adjuster about what the jerk tried to do, and how bad of a driver he is. Turns out that he got his policy cancelled because he wasn't listed as the driver for the vehicle, ha ha!

PS. Maybe you should build a cinderblock wall (if you can afford it), about 10 feet tall, and top it with barbed wire.

Well happy birthday to mersiepoo, my neighbor has not pointed a gun at me but I know him pretty well and that is not his way, he would have someone else do it, if it came to it. I think it all started because his wife began to like me, and after a few years of me not reciprocateing, she got pissed and he was jealous anyway's so that's one way one get hated by a neighbor. To add to it these are very base indiviusuals and with them it can sprial out of reason. I bet they can watch, what do you call it when cars go in circles for a very long time, nase car well they live for it, so they don't have much of a mind. I be live cat briar or Smilax rotundifolia L.
roundleaf greenbrier
While bird hunting I have gotten hung up in this stuff and it is hell to get out, it maybe ideal. In the mountains there is a plant called dead dog, and I have seen it, it is the thickest mass of low growing strong vines I have ever seen, I can see how it could kill a dog, he would not be able to hardly move once in the thick of it, the same could happen to a man, but a man could see that this stuff would be like quick sand.
I think a combination of catbrier and Mahona will do the trick. I would plant bamboo but he is a bad person and that would provoke him to spray herbicide. Cat briar and Mahona both have waxed leaves and he does not know mix a spray for this type of plant.

Don't you just love problem neighbors? I have dealt with one for the last 13 years. I have put up a fence because of his erratic sneaky behavior and am now searching for barrier plants like you, evergreens in my case, because I don't want this person to see me and vice versa.
Sorry can't help you much, I am zone 5... but I think that pyracantha (suggested above) has lots of nasty thorns though. They don't call it firethorn for nothing. And pretty berries.
Make sure you keep a log of all disgressions with this neighbor just in case anything dealing with this person escalates. I sympathize. Good luck.

Well I tell ya, I am sure going to know what to do when I am looking for a new home. If you are the outdoorsy type, look very hard at how close some one could build, to what you need as your personal space. If you smoke pot you don't want them smelling it, if you hate dogs fence your property, if you hate deer get a dog. If the dog digs up your plants get a bigger cat. I don't belive humans were ment to live in these cookey cutter neighborhoods . I don't care if you live in a million dollar house or a trailer it seem like the nuts are always just across the road. But we are all a special bread, meaning we are all homegrown naturalist to some degree, and that means we are to some degree an artist, and as an artist we are perticular about our enviroment. I will always remember that from now on. Nacar man how can anybody get into that, I guess they are just waiting to see someone get kill as they drive around in cirles, what a rush man. Yea I didn't spell chaeck a thing either, the question should always be can I pee in my back yard safely.

Now just hold on a minute, deshima! I'm not a knuckle-dragger, and I think Nascar is cool! I even have maybe 2/3 of a brain.

Too bad you're not in S. FLorida, I could recommend a number of nasty Zone 11 plants. Too bad you can't sow some manchineel in his yard, or that dense thing that has the recurved thorns out on the salt-water barrier-islands.

I take the death-threat thing very seriously. Last I was aware, if someone has threatened to harm you, is able to do it and acts like he's about to, then you can forcefully defend yourself as necessary. You may want to give serious consideration to the various self-defense options available to you; you don't want to be unprepared.

As to Nascar I guess you have to be there to appreciate it. You should see my Dad's house on Saturday beer and pop corn everywhere, National Association for Stock Car Auto Racing Anything But Stock. Doesn't bother me too much I just wished they all had time to do something more fulfilling,like lets all start a tree farm, but that may be considered work. As to self defense it could only come in the form of body armor and a Glock 39 w/a (green laser) pistol grip , plus, on going training to fire under extreme stress, or maybe I will just move. That is why I was talking about the fact that a humble gardener should be able to pee in his own back yard with out the police showing up. That is my new goal, and a place with out too much clay. I just learned today from my agg. agent that my USDA Zone had been update I think (due to globe warming) I am now in a Zone 7b, but it may even be an 8a but they don't want to scare the heck out of everybody. PS: Men make better gardeners!! OK I am gona put a joke at the end of all my posting from now on, and why is it that we cook so well? But I be darn if I can raise kids well, I can not get any of my girls to even hold a shovel.

That is why we moved into our neighborhood. 1 acre lots, so we are not right on top of our neighbor.

I would love to put up a cinderblock wall between us and our problem, but the county codes forbid it (I checked). No barb wire, razor wire, or electric fence either... dang...

Good luck.

So this guy is going threw or around your fence or is it that he is on a higher elevation and can look down on you.

I have a flowering quince in my yard that I am going to remove this spring because it's so thorny! I made the mistake of planting it next to a garden bench, ha! It grows moderately fast, and it suckers. The suckers are very hard to completely remove; I wasn't able to pull out the roots from a 1' tall, single branched sucker last Fall. It does have pretty blooms in the spring.

The Glock is nice, but I've always had a soft spot for the Dan Wesson .357 with the 2" barrel and a Pachmayer grip. ;)

This suggestion sounds perfect, I looked it up and I can thank of no better barrier. I removed a plant last spring for my mom and remembering that the blooms were pink like the picture of a quince that I just looked up seems to be the same plant. Then I went out side and I saw those two inch thorns and bam, I already have about four that survived the transplant. If the plant can handle such rough treatment I know it will be easy to propagate from some cutting. I would like to have a few more plant to build this hedge quickly,maybe I will check ebay. I wish we had this option for people in lower Zones, as this plant is a Zone 7 -9. My plants are all ready showing spring buding. My hydrangeas has not lost all of there leaves yet, so I think I am a Zone 9 so far this year.

.... was upset to read this post.... I like the poison ivy or poison oak vine idea the best (along with others - kind of intertwined so it's not so obvious). Thing is, can you handle it?

Carrie - (some people are allergic when it oils are released into the air).

Blackberries are good if the conditions are right. One of the nice wild varieties with tons of thorns!

The poster who suggested Sir Thomas Lipton was also on the right track -- lots of the tough old roses, and especialy rugosas, have a million thorns.

Good luck!

i have an issue with my neighbor. and as much as i'd like to do something awful like poison ivy or oak, i just don't want that because i have to deal with that too. and i can't see having thorns either.

leland cypress work good....but any plant is susceptible to herbicides. after a year or two, out of sight, out of mind.

That is all well and good but he comes into my yard at will and that is not acceptable given the nature of the beast. I have a huge investment in plant stock, a quick analogy: would you let some one just run there fingers though your safety deposit box that you do not trust. I need that lock box security : )

You need a dog! A nice big one that is trained to patrol the perimeter of your property.