that is terrible, but how do you know it was them? my dog was poisoned and we think that it was from drinking water from a dirty puddle, but it could have been anything (she was so good natured we used to let her off the lead).
try not to think of revenge, contact any relevant authority's if you have evidence of some one being responsible for these things!
Sugar in their car's gas tank!
That would be good, but gas tanks have locked caps on them these days, in case you hadn't noticed.
A potato or yam in the tail pipe will stall the car.
There's also the super glue in the lock treatment.
Putting a dead animal under or on the sill of an open window is good too.
Then, of course, there's arson when you really have to get them out.