in /scripts/sendmessage.inc.php and /scripts/mess_reply.inc.php
change
$mailer->set_subject('PMN_SUBJECT', array('BOARD_TITLE' => $config['COMMUNITY_TITLE'], 'FROMNAME' => $fromname));
to
$mailer->set_subject('PMN_SUBJECT', array('BOARD_TITLE' => $config['COMMUNITY_TITLE']));
then you can use something like:
%%BOARD_TITLE%%: Private Message from %%FROMNAME%%