why not just put vasoline on the windshield, its a real pain to get off, or you could put hot pepper powder in the vents so when he turns on the a/c or heater it blows it in his face

This is really good for a new car in which the guy doesnt know his gas mileage:

Get a 5 gallon gas can, and keep adding fuel to his vehicle without him knowing it. He'll think he is getting great mileage.

After a few weeks of doing that, start siphoning out gas instead of putting it in. Not all at once of course, prolly one gallon or two at a time.

Did this to a guy once and finally had to let him know about it cause I caught him on the phone making threats to his dealer.

lol...lol...........makes me lol

Wire the car's horn up to the blinkers... or perhaps even his speedometer (not sure how you'd do that one). scare the piss out of someone when they go to turn or go over a certain speed.

im getting my mechanic cousin on that shit stat