Actually, we do have captcha as well as email confirmation before accounts activate. The problem is spammers will always find techniques around this stuff no matter what you do. This stuff also has waves... bots focus on a site and then move on to another. The best thing is to flag and ignore. We are pretty ruthless about this stuff. We have some pretty great mods who are doing a great job keeping things clean. Keep them alerted... there's a lot to keep track of.
inXile's ruthless Leader of Scripters