Brandalism Any advertisement in public space that gives you no choice whether you see it or not is yours. It belongs to you. It's yours to take, rearrange and re-use. Asking for permission is like asking to keep a rock someone just threw at your head.
BanksyTraditional graffiti writers have a bunch of rules they like to stick to, and good luck to them, but I didn't become a graffiti artist so I could have somebody else tell me what to do. If you're the type who gets sentimental about people scribbling over your stuff, I suggest graffiti is probably not the right hobby for you.
Banksy