Yes you need to retaliate in order to feel better.
Have you thought about super glueing all the key locks on the cars. Trust me it works and sets like rock. Superglue has a nozzle that fits perfectly into the lock, just fill it and let it set. Another anoying one is put dog crap under the door handles and also put chili pepper in the front air vents.
Few more:-
Spray pepper spray under the door handles, even if it dries it will leave a residue on the hand.
Jam a potato in the exhaust pipe.
Leave a huge patch of oil under the car, they will think they have a major leak.
GOOD LUCK!!
I like that last one about the oil!