Put Vaseline on every door handle he has to touch (home, work, car, etc.) on the opposite side so he can't see it. Do this every day. He'll get so paranoid that he'll check every door handle he has to touch for a month after you've stopped. Not to mention it's fucking gross and hard as hell to clean off.
Run over to his house and duct tape his doors shut in the middle of the night. Don't use anything like caulking or glue as that will leave damage. You can do this on his car too, but I'm not sure if that would damage the paint or not.
Does he have a deck? If so, fill a disposable water bottle up with the shit that comes out of your garbage disposal and throw it under his deck. Wait a few weeks and some crazy shit will be living under there smelling the place up and making his deck unusable.
Post an ad on Cragslist showing a really nice couch or something for free and put his number in the ad. Say you're moving out of state and can't take it with you or something and that it has to be gone TODAY. He'll get like 100 calls in 2 hours.
Use a sharpie and scribble all over the windows of his car. I wouldn't do it on the actual car as removing the marker might damage the paint, but it certainly won't damage the windows if he does it right. He won't be able to see out his windows and hence won't be able to get to work or wherever he needs to go.
Save up like 10 fucking old ass computers and dump them on his front door at 1:00am. You can't get rid of those things unless you take 'em to the dump and pay tons of money to have them disposed of properly. Make sure they're REALLY old, or the Goodwill or whatever will probably take them.
Dress in all black and hide out in the bushes by his house in the middle of the night. As soon as you're sure he's asleep, fucking yell as loud as you can. Yell nonsense like you're crazy. Do this every night for a week. Make sure you're able to get away if he comes out and don't do this if he has neighbors close by. You don't want to be a dick to his neighbors, just to him.
Chew like 50 pieces of gum and put them on the ground right when he walks out of his door. What a fucking mess. But beware that you're going to be there a while as you'll need to chew each piece for at least 10-20 seconds. It's not the safest option.
