I've seen something really interesting while browsing phpBB3 mods :
Basically, this allows a moderator to add a "red box" containing a message that the user won't be able to edit. This is used to make notices etc, because I've had a lot of users removing what moderators put up in their messages, etc (they used to come back after a week or two, even ).
Another idea would be message locking. Let's say, if a moderator edits a message, then it's considered "locked" and can't be edited by the user again. That'd fix the problem from the first idea : an user could still delete his message's content, so that the message left by the moderator would not make any sense.
I really like that idea. I actually think phpBB3 has something like this, but even 6 years later, I still can't find willpower to install it . (I've installed it once, actually, but that was because it was a premodded board).
PS : what about we remove that "clean name" field ? This just confuses the hell out of people.