I'd make 'em clean it all off when they're caught -- and they will be, because they'll have to tell everyone about their brave deed.

Yeah, makin' 'em clean it COMPLETELY is a good idea.

And take it from someone who once did that sort of thing as part of his job, it's not easy. Takes a lot of elbow grease and work. And while they were doing it, I'd make 'em learn about PET.