I've implemented a new primary spambot filter on the forum as a protection from spam flooding.
The former primary filter in effect was the Q&A captcha question shown upon user's registration or posting. It has blocked the totality of the spam since it was installed. Before Q&A was in effect, spam posts were 3 times the regular traffic, and currently there are 100-200 spam post attempts per day. If all the spam would pass thru the barrier, the forum would get subverted.
The Q&A, still 100% effective, has now passed to the role of secondary barrier.
The new antispam filter is provided by http://www.projecthoneypot.org.
It's an interesting project collecting spambots' IPs into dynamic blacklists which can be queried by additional software. I've manually tested it with about 10 IPs harvested from the forum's webserver log file, and it correctly distinguished all the spambots from the users. Projecthoneypot claims it has one false-positive every 50 millions queries.
I've also performed additional setup to apply as a volunteer to let this forum automatically report spambots IPs to the project's blacklist.
Let me know if the new filter triggers against you.
More info on the spam system: