I say we.. or I suppose you, since I'm not really a figure of authority, allow him a change, but just this once. And if he does it again without permission, temp-ban him for a week or so. Then if he does it again, temp-ban him for a month. And if he does it yet again, perma-ban him. I may sound kind of like an @$$ here, but it's just something you gotta do, ya know?