If it were me, and I was in your position (non-confrontational), I'd have only one real course of action.

I'd start telling everyone (in a joking and pacifist sort of way) that "some fool is sooooooo jealous of you/your car that he actually went all the way over to it to spit on it to try to make you mad. How juvenile, are we really still in the 5th grade?!?! How lame!"

Sooner or later it will make it to the person responsible and his friends, and I'm sure the latter will be making fun of the idiot who did it!