When moderators send a message, the current system only allows you to post a single reply, then prevents you from following up again in any way until a moderator responds first.
This is bad because not only are there plenty of reasons that someone may need to post again, but it also allows corrupt mods to freeze out a user by simply not posting a response, thus completely shutting the user up for no good reason.
I cannot conceive what legitimate reason there could be to prevent users from replying again, but if the concern is of spam, then simply put a timer on it that prevents them from posting responses too fast.
There should be an edit button to allow a user to modify their previous post. Not only would that allow them to add additional information, but it would also let them clean up their previous responses which might be a little heated and rushed considering the nature of most moderator communications.