Read it again - he isn't saying Fuck The Advertisers. He's saying feel free to deface any advert in a public space, something I wholeheartedly agree with. If you thought about it, I'm not sure any of you could disagree with it either - do you really support the legal system which allows butthurt corporates to have your domains taken off you just because they happen to be similar to theirs?
Banksy blatantly isn't anti-capitalist (even though he cynically positions his messages that way), because otherwise he wouldn't have accepted so much money for his artwork.