The captcha prevents spammers from flooding the forum. It is only nessesary on new accounts, though, unil they make a legitimate post, and get manually moved from the 'new user' group to 'user'.
It sems that is somewhat annoyng to a user. Well, could you activate for trolls or other monsters? .. you know just troll them back.
Can you handle double test for someone ?
And what about triple ?
I would really know what is the longest captcha chain that a troll could solve. You know they say just "troll toll", but never uderestimate them.