Nothing really, guessing it won't do a great deal of damage if they are just using their hands. Chase after them like a crazy old witch the next time they do it.
I've been at a friends house and her dad kindly brushed the snow of my windows with a house brush. I nearly died..
failing that, when they are not about go out and remove all the snow off the car your self that way they wont be trying to collect snow of it, simples my dear
sneak into your car when they aint looking and wait till they do it again then as they touch your car beep the horn lol theyl **** themselves, might not stop them but will be funny