Another idea would be instead of a captcha to use a set of standardized questions, like "which command do you type to get root on the N900?" (answer: "sudo gainroot") and such. Obviously one time when registering.

This would require some effort, but even for new users who have a genuine interest in joining the community but lack the knowledge it would motivate them to search for the answers, which is always a good thing in any forum.

Costs nothing, protects from "spam monkeys" and encourages the right attitude in a forum