Auths for subforums by usergroups

[RedFoxy]

Hi all!
In my forums I've a lot of usergroups, I use them in order to assign auths for private subforums, the main usergroups REGISTERED USER has all auths for writing and reading on the0 whole forum (except for the private subforum), now if an user needs the auths to see a private forum, I'll add him in the usergroup of the private subforum.

That system it's ok to ADD auths, but now I need to REMOVE auths to a lot of users who have been banned from some public subforums, would it be possible? And, if yes, how could I do it?

I made a new usergroup and I then removed all auths for the subforums, on that usergroup, but the user can still read and write in the subforums because he is of course a member of the main REGISTERED USER group...

Help me plz -.-