Deactivate user feature
Right now, it's difficult to deactivate an account but not delete it. You can delete them from all user groups but this doesn't suppress log-in functionality.
One use-case is where you want to remove someone's access but retain history of their edits, etc. At any rate, an activate/deactivate feature is pretty common in other user management systems.
Group listings in the CMS might hide deactivated accounts by default, and/or they might be highlighted differently.
Log-in attempts should say "Sorry, your account has been deactivated".