See your notifications.
Banned users cannot post and bans can be set to automatically expire after a set time. Perhaps we do not understand precisely what you want to do, because somewhere in the three or four solutions you have been given, there must be something that will work for you. Hundreds of thousands of boards operate with this same set of tools.
What group? Unless you expect hundreds of miscreants, ban the user, put a time limit on the ban, and give Guests a Read-only access role. Done and done.