That's why most forum softwares allow option of new users not having ability to send PMs or emails until they've hit a certain post count -- spammers will get blocked within their first 10 posts on a forum (if not less) - so the possibility of users configured that way being able to spam the yaf_pmessage.aspx file would only be if you allow 'untrusted' members ability to send PMs or Emails...
Well, I think would be more user friendly to create PM sending frequency (for exam allow to send 1 message per 10 min). Or 10 msg per day to different users.
Manually approving absolutely can not prevent from spammers. If a forum has a few thousand users the spammer can make provide a fake activity as a usual user. Аfter he obtain an approval, he will make a spam in a short time.