You could always get a handful of big wheel weights and add them randomly to the inside of his wheels one night.
Another good one is to take an air tank and take the filler off of it. Then fill it half full of water. Put the filler back on, and refill with compressed air. One dark night you let the air out of his tires, and then refill from your air tank while holding it upside down. This will push the water into the tire, and he will never be able to drive it due to balance issues, and the tire store won't be able to rebalance them without removing the tire from the rim and drying it out. This works even better when it is below freezing, because he suddenly has a big block of ice in the bottom of his tire.
Yea.. I know I am new here, but I absolutely hate people that mess with someone else's vehicles.