I agree with Josh. We have a "time-out" group set up that takes away access to post in any forum (they still have read access to most forums, but not all). They don't have access to PM or post. This is what we do with people who we feel need some time to "cool off". We pull them out of all the groups they are currently in (including users) and stick them in the time-out group for a few days. We haven't had the need for a forum like you've mentioned, but I actually think that's a great idea. You'd just set up a new forum with read/write access to admin., mods, and your banned group. Just make sure you don't truly ban them, or they won't be able to post.
Just create another group called "suspended" or something.