I think I'm going to smash the car windows of a guy who keeps sending my girlfriend panties
What gave you the right to smash in the guy's windows?
Well, this guy is a stupid shit.
I can understand your resentment towards a guy who sends your gf panties, but breaking his car windows is a bad idea. You will be arrested for vandalism if you do.
But he keeps harassing her with those panties
What that guy is doing is real creepy, and I'm happy you are very protective of your girlfriend. But resorting to this violence can be criminal, think of how your girlfriend would think of you smashing up a car. Why not silently let the authorities handle it, so he can get a proper sense of justice and you two get to live peacefully?