To the people not wanting their car randomly tagged. Maybe only tag a car with an NS sticker that is signed by the user already. If you see a sticker not signed, sing yours and leave it with the backing under their windshield wiper so they can place it.
Obviously don't be a dick and tag a car in the windshield or on the exhaust or something retarded like that.
I figure if most of NS participated, this could get fun.