Everyone has probably had the neighbors from hell at some point or another. If you can't move or make them move at least you can cause them frustration. ITT we discuss things to do to the neighbors that will waste their time & money & inconvenience them. All while remaining anonymous.
So, far this is what I've come up with:
Puncture the tires on their vehicle with something, maybe nails or screws. They will have to buy all new tires & as an added bonus if they park a bunch of vehicles in their driveway, by damaging the the tires of the vehicle on the end, the vehicle that's boxed in can't be moved.
In the winter, toss rock salt on the icy sidewalks, making sure to get some one the neighbors yard. Salt not only destroys grass but the dirt, too. Even if they reseed it or put new sod down next spring/summer, it won't take.
Any other ideas?