How to prevent bots from subscribing to forums?

Spam filter box.
Since I wrote my previous article on how to fight spam in phpbb forums, we have seen a reduction in spammers signups on our plastic surgery forum, but we are still getting two types of spammers:

    Spammers who pretend to be regular folks (they keep changing their user names so they have no digital footprints on the web and may even list their hobbies etc. to make them appear more real) and once you approve them, they start the spamming process. The trick to deal with these crooks is to delete their messages immediately and ban them.
    Spammers who never visit the website but use bots to signup. These bots are smart enough to fool the captcha feature of phpBB. We found a very simple way to fight these. As you see above, we created a simple field (by changing an existing field for ICQ number) on the signup page asking the registrant to enter “HUMAN” in the box. If you are a human being, you will enter it, and when our moderator views it in the control panel, this person will be approved. However, a bot that is designed to fill this field with the ICQ number, will be unable to do so. My research shows that some forum owners have even put the code word on a different page – something that a human will find with no problem, but a bot can’t.

I am not sure if we can make our forums spam-free, but these simple tricks will keep most of them away.