People will think I'm being harsh, but, I'd deck 'em!!! If anybody even so much as touched off it. My neighbour was trying to teach his 2 year old to write on my car cause she used do it on his Carina II all the time, told him if I found even one speck on the car he'd be forking out for a full re-spray, didnt take long to stop her trying to write on the car then! And he knows I would have made him pay too, I wasnt joking.
If I were you I'd give out to them, they're only going to think its ok then if you dont, and that its funny. They need to know after once that its not right and they gonna suffer the consequences if its done again.