I agree that account registration is too easy. There needs to be two (or more) factor authentication when signing up. You guys already have the email verification, which is good, but easily circumvented on its own. A captcha (like above) would solve the problem.
I haven't really looked into it so I'm unsure if there's any free solutions, but there's bound to be. If not I'm sure you guys could get a few bucks out of some of us maybe?
EDIT: Looks like it is free http://www.captcha.net/ This appears to be a simple enough captcha implementation method: http://www.captcha.tv/captcha-explained.html though it's php and these forums are asp so I'm unsure if it'll be supported.
According to this, the administrator should have the ability to enable captcha in the forum settings: http://activeforums.codeplex.com/discussions/433314
The forum software is open source and community driven: http://activeforums.org/ https://github.com/ActiveForums/ActiveForums so it wouldn't be hard getting it sussed out regardless
Just to be clear I am not telling you lot how to run your website, just trying to help.