Manual:$wgRevokePermissions

Details
$wgGroupPermissions allows to set permissions for users. $wgRevokePermissions allows to revoke any of those permissions from users in each group. 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 group), 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.

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