If there's a rogue OpenID server, it could be banned, the whole kit and kaboodle, I believe. Though LJ's full of spammers the poor thing, and it would not be too difficult for LJ spammers to make the leap in that way. Though LJ would probably be glad to suspend the little bastards should they do so. It could be more work, but I think it would be worth a trial.

If it happens, it should come with settings to differentiate posting permissions: I think a lot of community admins would be fine with it if they could set all OpenID user posts to moderated and give regular users open posting.