Spambot countermeasures
Here you can select and configure plugins, which are designed to block automated form submissions by spambots. These plugins typically work by challenging the user with a CAPTCHA, a test which is designed to be difficult for computers to solve.
For additional (and possibly better) anti-spam plugins, visit the phpBB.com Extensions Database. For more information on preventing spam on your board, visit the phpBB.com Knowledge Base.