Kills spam-bots, leaves humans standing. No CAPTCHAS, no math questions, no passwords, just spam blocking that stops spam-bots dead in their tracks.
BotBlocker should not interfere with your form customizations, whether you have added fields or removed them. Barring extensive customizations to the core of the comment system, there should be no issues.
BotBlocker works by hooking into WordPress's comment system and should play OK with most plugins, however there is no guarantee.