I am unable to comment on who may or may not be Banksy, but anyone described as 'good at drawing' doesn't sound like Banksy to me.
BanksyBrandalism 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