Rather than do a hack, I created a special forum for the banned:
Village of the Banned
You're here because you were banned. If you need clarification start a thread and a moderator will respond.
Permissions are set so it can only be viewed by banned members, admins, mods & supermods, and when a member is banned that's the ONLY forum they can view so they know they're banned. Banned members can only see and reply to their own threads. I rarely get email involving bannings since I set this up 3 months ago. You can set the policy anyway that's best for you, but mine is that only the mod who did the actual banning should respond, and if they're not around an admin will handle it.
Can you tell me the exact steps you went through to make this possible? Each time I try to do this I make the forum etc, and when I ban someone they get the no permission screen, even though the permissions are set. Does this apply to temp bans, or people I've specifically moved to the banned usergroup? The reason I'm asking is that mods can't specifically ban people, they can only temp ban them