Manual:$wgRevokePermissions

Details
$wgGroupPermissions allows to set permissions for user groups. $wgRevokePermissions allows to revoke any of those permissions. Revoking a right with $wgRevokePermissions takes precedence over granting it with $wgGroupPermissions: If the right is revoked (and if only for one of the user's groups), then he will not have it - also, if it is granted to his other groups and also, if it is not revoked from his other groups.


 * 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. A good use-case for this setting is in conjunction with $wgAutopromote and APCOND_BLOCKED to further restrict the rights of blocked users. Another use-case could be the creation of other "blocked groups" where a sysop can (via $wgAddGroups) add a user to a group to still allow them to edit pages, but prevent them from being able to move pages.

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