Manual:$wgRevokePermissions

Details
Permission keys revoked from users in each group.


 * Example:

 Edit the user interface ( editinterface ) 
 * Result is (when viewing Special:ListGroupRights):

This acts the same way as $wgGroupPermissions, except that if the user is in a group here, the permission will be removed from them.

Improperly setting this could mean that your users will be unable to perform certain essential tasks, so use at your own risk!