I can't take credit for this one, it's my friends idea...

here's supposedly a product called "glass etch" or something like that...and it eats away at glass...
the idea is to carry around a squirtgun willed with the stuff, if anyone messes with you, simply ride up to them, and spray it on their windshield...I doubt anyone would take offense to being squirted by a squirtgun, and most likely they'll think you to be a wuss and just laugh at you...
meanwhile they'll use their windshield wipers, thus spreading the glass etch all over the windshield...the next day the'll arrive at their car with their windshield completely F*CKED UP!

it might be a delayed gratification, but you'll smile the whole time