[Stadler]

With vB3 you could do the following:
Add a new group with restricted permissions (e. g. they aren't allowed to post) Select Yes on Usergroup is a banned group (or however its called in english ) and them simply place a time limited ban over them moving them into the restricted group and they will be moved back to their 'old' group, after that time limit has expired.

I haven't tested it myself, but that should do the trick

PS: Don't forget to check their permissions for your private forums unless you want to give them access to one of them