Roundup will kill them, but for very large clumps, it may take a couple of doses. You'll have to wait until much of the foliage has grown in, and soak them down good, with a strong dilution. Of course, you'll still have to cut down the dead foliage, and should dig up the dead crowns.

Another possibility is to let area landscapers and gardeners know you want to be rid of the grasses. If you were closer to me, I'd remove them just to get the plants.

I have dug up several large clumps with the help of a friend. It's a good workout, varying in difficulty depending on the species and cultivar, but very doable by two women. I'm a 62 year old woman and my helper was 53. The toughest one to remove/move was Molinia 'Skyracer'. Use an axe to cut up the clumps once they're loose.

I think Donn had an excellent idea! And, one that will cause you the least amount of work.
- Try one of your small local nurseries or landscapers - they may come & dig the grass out for you - happy to have the plants.
- Put a sign up 'Free Ornamental Grasses for the digging'
- Post a note on the plant exchange forum - You might get rid of the grasses & make a new friend :o)

As Donn says, killing the grass with herbicides will still leave you with the root to dig out - at that point you might as well dig out the live plant & leave the herbicides on the shelf! IMHO.
A.

Here is a link that might be useful: Plant exchange

I just dug up a large Miscanthus and it was not easy... the plant was so wide I could not get the shovel far enough under the plant to cut the roots easily, and the root ball and top weigh 70-80lbs and is hard to get a grip on... so it can be quite the task. Killing it won't make it easier, unless you just let it rot in the ground which would be an eyesore

So what do you do with the dug out plant? It is too heavy for the yard waste people... I'll try to cut it in pieces with a saw

This is not to make light of a daunting task, but it does show the importance of planting according to mature size when planning a garden.

As for the task at hand, digging is probably the better option to herbicides, which will likely leave a root system that will require digging and working the ground for replanting regardless.

Best option suggested is the 'find a friend'. If someone wants the grass clumps badly enough, they'll do the digging.

It pains me to say this, because I used to really love these plants, but here in Virginia these grasses are becoming a major problem, achieving invasive status in many areas. So when I'm asked this question in VA, my response is either 1) Roundup or 2) hack 'em up and compost 'em (if your compost gets hot enough to kill the seeds) or 3) trash 'em (our trash goes to an incinerator, so that stops the problem right there!)

an easy way to get rid of them is to cut them down in the fall, and giving them a really good drink of water right before it freezes---amazing how easy they come out in the spring if you can get the crown wet and make it stand in the ice! I goofed and cut a couple down one fall and the fall rains took care of the rest for me---lesson learned--they say winter interest on them for a good reason.

I just bought a house and there is a stand of some kind of ornamental grass that takes up an area of about 100 square feet. It is not pampas, just some tall woody grass. Also, it is on a gentle slope. Is roundup my best option? Should I use hedge clippers and take it down to stubble first?

You should start a thread for this and a picture would be good.

A sturdy tool with a sharp blade helps. I dug out the last my my Miscanthus this spring with a DeWit Spork:

https://www.youtube.com/watch?v=OsqAYTUmkpQ

First I cut the grass down as short as possible with a Hori knife, then I was able to chop right down into the plant with the spork and pull out sections at a time -- much easier than trying to uproot the whole thing at once. After that I dug down farther to get as many roots as I could find, knocked the dirt off as best I could, and left them to dry in the sun. I ended up throwing away a few pieces with each yard waste bag I put out so each bag wouldn't be too heavy.

This tool is pretty great for chopping down through stuff that isn't too woody.

I just might have to get one of those. It looks like it'd do a job and I think I'd look pretty tuff walking around the yard with it.

It does impress the passers-by for sure. I also walk about the yard quite a bit with a hori knife. If I see, say, a family with small children coming down the sidewalk, I immediately try to find something to cut or dig out so they can see it's a guy with a garden tool and not just a lunatic with a big knife.

When I was growing cereal rye as a cover crop I thought about getting a scythe:

http://scythesupply.com

but it seemed like overkill for a tiny yard like mine.