The forum has now been updated to version 3.8 to fix a few security holes. Ant-spam stuff via Akismet is now built in and there are additional options when moderating threads (mark as spam / ban user) which should make it easier to control spammer.
The new release also includes the ReCAPATCHA system and other things. I've not looked into in to much detail yet though.
Please let me know if you come across any other problems.