I can't think of a decent reason to allow editing audit messages (like 'n messages moved to room' or topic changes). I have abused it for fun before but it really has no purpose other than joking or obfuscating your actions. For that I think that such messages should be untouchable. Please make them so.
A simple way to fix would be to make these messages work like the ownership change messages. Those are owned by @Feeds, and therefore no one can edit them (mods excluded, of course; s/m/g/).