Hi all,

In connection with the introduction of the new antispam filter - Merits - I would like to summarize what antispam filters I have found on the Bitcointalk forum. And at the end of this post, I'll also list some other antispam filtering methods that are commonly used on forums.

So, what I've found to be currently used on Bitcointalk forum :

Flood control forces users to wait for an interval between making posts to the forum, thus preventing spambots from overwhelming the forum with repeated spam messages. The intervals decrease with increased rank of user.
CAPTCHA (visual confirmation) on forum registration page - it can help prevent spambots from carrying out automated registrations. For Bitcointalk forum, advanced CAPTCHA system is used called "Recaptcha" (images recognition), as simple captcha systems which display alphanumeric characters have proven vulnerable to optical character recognition software but those that scramble the characters appear to be far more effective.
Confirmation e-mails to users who registered, with activation code/link.
Searching engine has got an interval, after which you can search again. This interval decreases with increasing user rank.
Trust system - spammers can be marked with negative trust.
And the new Merit system

And what are some other techniques for avoiding, removing, and mitigating forum spam, used in other forums?
Some of them might be used on Bitcointalk forum as well, but they are not visible to users, so hard to say..

Blacklisting services such as fspamlist, StopForumSpam and BotScout keep databases of IP addresses, usernames and e-mail addresses used to post spam or register forum accounts. Forum software can query these lists and either deny posts or registration, or submit the request for human moderation.
Textual confirmation as an alternative to CAPTCHA in which the user answers one or more random questions to prove that he/she is not a spambot.
Manual registration approval by administrators for each account.
Denial of registration from certain domains that are a major source of spambots, or even domain extensions
Manual examination of new registrants for several indicators. Spammers often delay email confirmation of several hours, while humans will confirm promptly. Spambots tend to create relatively noisy user names in order to ensure uniqueness.
Using a search engine to investigate usernames for hits as recognized spambots on other forums.
Changing technical details of the forum software to confuse bots
Blocking posts or registrations that contain certain blacklisted words.
Monitoring IPs used by untrusted posters, like anonymous posts or newly registered users. A useful technique for proactive detection of well-known spammer proxies is to query a search engine for this IP. It will - show up on pages that specialize in the listing of proxies.

Hopefully you find this information interesting and useful. I wonder if you know about any other antispam filtering method used on Bitcointalk? Or any other good filtering method used on other forums, that is not mentioned above?

Have a good day !
Boris