You could also get a very close range bio-electrical field monitor, and attatch it to both of the front wheels, make the range about 4" yes inches, that way, when someone reaches near your wheels to chalk them, the sensor would set off a buzzer you could wear like a pager just get a long range transmitter, so you could get it if your a good distance away, and like I said, have it hooked up to a pager motor, so when someone comes within 4" of your front wheels, it'll go off, giving you enough time to check to see if it was the people who do that stuff...
You could also mount the monitor in the wheel well or just above it, so it wouldn't set off the buzzer while you were in the car.