LOL... just joking!

Have you searched here? http://www.phpbbhacks.com/

Anyway... I asked to a friend of mine and he told me it's quite difficult to do without using the mysql db directly.
In the db you should find a table named "phpbb_users" in which each registered user has an own progressive ID. Here you can find the Anonimous user which has ID set to -1. If you delete it, any new user that attempts to register will prompt a message that he/she is already registered. This means that the Administrato can't create new users too!! So you will need to manually edit the db. I'm not sure this procedure can work properly but it could be a tip...