Prevent php regex, someone using private.php maybe using they bot spammer script

[basketmen]

like in first post, Its not about the pm's settings, they can set the bot to send only x pm's perhour too if we change it, and create new members/new ip address even we already ban it

the best example is in webmaster central forum, forums.digitalpoint.com, if you are the member there, sometime you will get pm's like this from spammers

it is better if there are way to block private.php page used by cross linking, only allowed real user

please share if anyone know how to make private.php page more secure