Yes - but putting them into the banned group, means they only see the forums that that group is allowed to see.
The problem comes when person A must not see forum 5 and person B can't post to forum 7 - these two can't be in the same banned group as they need different pemissions.
Whereas if there was a method for a ban list for each forum (maybe username and IP) then moderators for those forums could decide who to ban and for how long.
The main use for this system would be forums that hosted club or community forums, where the moderators wanted to have some say on who can and can't post in their little section.
This would save these clubs or groups going to one of the many free forum hosters and stay and have a ubbt forum
