The passifloras may work - mine didn't really get going until their second year, then they really took off. They will require some kind of support though - mine grow on a lattice and over a garden arch. They also die down to the ground each winter and take a bit of time in the spring before they "wake up." After that, you'd better keep moving or else you'll be overgrown.

If you want to be mean, heavy, self supporting vines to consider might be wisteria, virginia creeper, and/or trumpet vine, or maybe even grapes. These are very aggressive and they will hold onto adjacent buildings/walls and they do grow trunks sufficient to at least assist in supporting the vine. They can also be damaging to the buildings they're growing against, and that might open you up to repair costs to their building - so consider your choices here carefully.

I have the same problem with our neighbours building a deck so high right along the property line that they enjoy a view of our entire back yard. Also their loud voices carry so that we never have any quiet or privacy. It took me 20 years to build my large garden space and in the space of 6 months I lost all my privacy.
If you have space along your property line consider plantings in layers. You can do vines(golden hops are vigorous in my zone 4)then a layer of tall upright but compact evergreens that will shoot up. Then add a layer of tall shrubs or tall bamboo or tall grasses. Consider the taller miscanthus or Giant Wild Rye. I am not sure how much room you have to work with. However, with the layering you can at least hide behind the mixed plantings when you are working in your yard. Good luck.

Thanks for your responses guys. After some thought, I went with Campsis radicans Madame Galen (hybrid trumpet vine/ creeper). It clings, it grows like crazy, and it has lovely flowers. Even if they remove it from their side, it will grow back. And I do not think that I have to worry about damage to their extension, as they should prune the vine that climbs over to their property or pay the price. Besides, their extension is illegal, so they don't have much of a leg to stand upon. I did consider Virginia creeper too, but I think Campsis has nice flowers. Don't Wisteria / grapes need support?
I don't have much space to do the plantings in layers approach. The garden is 17' x 35'. I am also going to grow the passion vines. The neighbor incidentally has grape vines that provide some privacy (and intrude upon our property), but they are not in the best location to shield their ugly structure.