Troller and Sol,
I have thought about the same thing, but in a less lethal or potentially liable way. Perhaps leaving a small box or wallet in view on the seat. When they open it a dye packet explodes, similar to at a bank. In addition, just for our pleasure, a good dose of pepper spray also nails them. While it may not have the ultimate effect I would like, it may still deter them a little and at least make them identifiable for a short period.