Extension:DisableAccount

The DisableAccount extension allows administrators to disable individual users' wiki accounts.

Disabling an account will remove this user's password and email, and place it in an "inactive" user group. It is not reversible without system administrator intervention. To re-enable an account, remove it from the inactive user group, and reset its password using Manual:changePassword.php.

Installation
The second line above assumes you want to give the  permission to users in the bureaucrat user group. Replace bureaucrat in the LocalSettings.php lines with the appropriate other user group, if you wish to use a different configuration. Obviously, the permission should not be given to user groups that are automatically assigned or widely populated.