if your willing to completely destroy his car, which i doubt you are, you could put saw dust in the petrol tank. i don't know if anyones metioned that before, i havnt read all the posts.
also if you were careful and accurate enough you could use paint stripper, when he comes back to his car, it won't look like his anymore. of course you would have to wash all the paint off the floor.
i wouldn't advise trying it though.haha.