It is a public forum, but only admin people can post. I've change unregistered users to "Read" only, and I've manually removed the "Post Message" button from the HTML code of the Prologue/Epilogue, but the button is still there. (Mind you, UNregistered visitors just get the "You can't post" message, but I'd like to remove the button from the public side.
In the Security Section of your Admin page, set registered users to Reply. Reply will allow them to reply to messages you post but they will not be able to start new threads.