1) Not possible in the stock UBB - sorry..
2) Just remove all the forums from them and they won't show up except in the control panel. You can't really delete them safely (although it is a lot less dangerous than #3)
3) DO NOT EVER TRY TO DELETE A FORUM. NEVER EVER delete a ForumX directory by hand, or edit forums.cgi DOING SO WILL CAUSE MAJOR DAMAGE TO YOUR UBB rendering it UNSUPPORTABLE on the forums (and Infopop will not be pleased to fix it).

Deleting forums IS possible in UBB6 and works quite well. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />

Graeme