[Martin64]

The easiest way to do that would be to open mod/user.php and...

FIND:

if ($usergroupid==6 or $usergroupid==5 or $usergroupid==7)  {
		echo "<p>You can't add admins or mods!</p>";
		exit;
	}

REPLACE WITH:

if ($usergroupid==6 or $usergroupid==5 or $usergroupid==7 or $usergroupid==XX)  {
		echo "<p>You can't add admins or mods!</p>";
		exit;
	}

Replace the XX with the ID of the usergroup you don't want your mods to be able to add users to.