technically... incremental permissions reversed...

OK, we have more than 800 forums on cf.com, and i want to reverse access permissions now that the administration is in my hands... instead of granting access to specific forums, i'd like to ban specific usergroups from these forums, so if a user is troublemaking in a forum, i just put that user in the secondary group X and he does not have access anymore...
at the first, the user is given a secondary group that identify his ownership to a congregation, and then that congregation have access to specific forums... but this user is a troublemaker in One section of that congregation (let's say the debates) and i have a usergroup to use that ban members of it from posting in the debates...

i tried it with the default vB configuration, but as the system is incremental, once you give access, you can't block it with a secondary usergroup...

is there someone having a solution for that, or i will have to deal with a hand-made code ?!