This has always struck me as a strange thing to not have. We have the ability to set things as Community Wiki at the start, and the ability to switch over to Community Wiki after something has been posted. Unfortunately there is no way to move something that is Community Wiki to a traditional post. I haven't even seen a method for this in the moderator tools, and I believe I remember reading something by Jeff saying that it was not possible at this time.
That said, the request is pretty simple. Allow for Community Wiki posts to get reversed back into regular posts. This would include making it an option in the mod tools.