The old Banana in the Tail Pipe works well.
Simple take a new banana or two and shove it into someone's exhaust pipe. This creates a back pressure and the car will stall out.
Variation : Use a hot water bottle or ladies douche bag, clamp the open end of the water bag to the end of the exhaust tail pipe, lift the bag up and loosely duck tape it to the pipe so the car owner cannot see it hanging down. Once the engine starts the pressure will build up and cause the bag to inflate and explode scaring the hell out of the owner.
Disclaimer : These are not my ideas, and this behavior is most likely illegal.