I think the easiest way to avoid this is to remove the red, and blue chat color options. I don't think people would be too disappointed to lose out on only two colors, if it meant this could be avoided.
At the same time though, impersonating a moderator is against the rules, and players can/will be punished for doing so. They really can't do that much damage, so I'm not sure how big of a deal it would end up being. They can pretend they'll ban, or kick someone, but they really can't anyway, so they can't do too much damage.
They do it, take their punishment, and the ordeal is over.