Currently, as admins we can censor words, that replace a word with other word. But that's different than banning a word, or in this case, a domain. If I censor a domain, a image would still appears in the post but broken because I replaced that word with another. The behaviour I would suggest is just to alert the user about the banned domain until the user fix it in his/her post.
I think this could be a very handy feature for any board administrator.