Wow... a few really childish responses here, I'm hoping a lot of them are more tongue-in-cheek than they seem. What lesson are you going to teach them by any form of vandalism? You'll teach them that there are jerks who vandalize cars for no good reason, they won't understand the context.
Did this actually cause you any harm? The cagers here are self-centered and something should be said to them. How about a professionally worded note left under their wiper blade? How about calling the local PD like others have said? Think about it, they're probably not bike riders so they may just be completely ignorant. People who don't ride don't realize the ramifications of their driving/parking on bikes. Politely inform them and they'll be likely to listen, move their car, and maybe even apologize. You have nothing to lose by this kind of response, you've got a whole lot to lose if you resort to any of the above-mentioned vandalism (including possibly arrest or getting the snot beat out of you if they see you).
Water off a duck's back man... keep on riding and keep the smile on your face, you've already given this too much of your energy.