You may want to look around for better free message board software. I've had many varieties of attacks on PHPBB web sites I host. PHPBB will always be a prime target and frankly it has gotten too bloated anyway.
I am looking forward to the eventual (long overdue) 1.3 release of punBB software. It is leaner, much faster than PHPBB, and has much better security.
I will be converting my PHPBB sites to PUNBB as soon as version 1.3 comes out.
http://punbb.org
http://www.punres.org
.:.