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

[basketmen]

Hi guys,

about few days a go, my members got a lot enough spam private messages from few spammer members, its looks like there is a bot spam script outthere, to search members username and send spam private messages, maybe using php regex

is there a way to prevent private.php page by accessing from other server, so only real user that can accessing the page? maybe like prevent hotlinking image

Nb.
I already set member can only send 5 pm at once. But they can still sending constant pm. I get simillar pm from forums.digitalpoint.com few times before, even my account dont have post yet there

Its not about the pm's settings, but how to prevent this private.php page, and other page that can accessed and abused by script from other server maybe using regex

please share your knowledge guys