[alexhertz]

Captcha won’t help. All spam software is trained for captchas. If all bbpress forum would use the same captcha, spammers will quickly adapt. Email verification is also useless: every decent spam package allows for confirmation of mail links.

Try more of Turing test. One option is a two-level confirmation: a mail link AND after it is clicked – a captcha. Something new that spam software doesn’t expect.

Or send an email with password and OPT-OUT (rather than confirmation) link. Automated software will click the link, and opt out of the forum.

Or send email verification with text to be typed into url window rather than a link.

Or, send confirmation email which must be confirmed by reply-to rather then clicking a link. Still better, reply-to and type something in the subject line.