"how do we know that Banksy's art is all by the same person?" We don't. "What prevents others to publish art under Banksy's name (with or without approval)?"

"how do we know that Banksy's art is all by the same person?" We don't. "What prevents others to publish art under Banksy's name (with or without approval)?" Whoever owns or manages Banksy's brand and instagram profile.