I've been dealing with cases involving ex-boyfriends and ex-husbands messing with the automobiles of their former significant others, or their former significant others' new beaus.
This is beyond feather-legged. To paraphrase Vincent from Pulp Fiction: You don't mess with another man's ride.
If you're such a coward that you can't face the guy directly to address your aggression toward him, then you have other issues worth dealing with. Dude. She left you. Move the hell on. Keying his paintjob or slashing her tires isn't going to do you any good at all. It might just land your butt in jail, if I can catch you.
And if the car's owner catches you, I can't promise you that they won't shoot your butt. It happens.