Banned users don't have any permissions whatsoever.
Once a user is banned, they no longer have access to the board content or any of the functionality. They will just see a message stating they have been banned, along with any reason entered at the time of the ban being placed.
Banned users are not included in the member count and do not show in the member list or search. In addition, the profile pages of banned users are only accessible by Administrators.
By default there is no outward indication that a member has been banned. If desired, a user group can be created specifically for this purpose, with the appropriate display styling priority set to ensure it overrides any other groups the user is a member of.