How to fight SPAM in forums?

Picture of a forum home page.
Fighting spam is hard and we are currently fighting a lot of it on our plastic surgery forum. While, not 100% effective (nothing is), here are a few things that you may want to use in keeping spammers away from your phpBB forums.

    All new members must be approved by the administrator or moderator before they are accepted.
    Delete the website URL feature in the member profile – this is the main reason spammers sign up. Modify the code so that all links have the “no follow” tag. While many spammers will lose interest in signing up, it might still be of interest to others who want to steal the traffic. Here is the step-by-step explanation on how to change the code for both.
    When you implement the two, then post a message like this on your signup page: Important message for website owners: (1) This forum does not allow URL in your profile. (2) All links have no-follow tag. (3) External links are not allowed.
    Most spammers do not come themselves to signup and will use automated systems to do it for them. Here is how to find them. When you go to approve them, do a Google search on both username and email address. Chances are you will see their Spam footprints. If not, send an email to the address and ask the person to respond. If you hear from a human, you can approve; otherwise, you will get an error message since spammers typically use fake addresses to signup.
    Despite the above, there is no option other than to watch all activity, delete any spammy messages immediately, and ban the member permanently.

Update: Since writing this post, we have had some more experience dealing with spammers. Read my next post on how to fight bots signing up on forums.