You would need to alter mess_reply.php and sendmessage.php language files to something like...

$ubbt_lang['PRIV_BODY_R'] = "###PLEASE DO NOT REPLY TO THIS EMAIL###\n\nHello, You have received a private message on the boards at {$config['title']}. Please login at {$config['phpurl']}/login.php to view it. Thank you \n\n------------------------\n\n";