spam user registration

My site (http://ispeakx.com) seems to have quite a few fake user registration with email address from : mymail-in.net, mail.ru, gmail, etc. I use captcha math questions, which does not stop this. What is the best approach available in Drupal?
I also try to make two CCK taxonomy fields as required during registration (see http://ispeakx.com/user/register), hoping it can deter some spams. However it doesn't behave as I'd like. If you assign a category to a content type, the default will be "please select". On the other hand, if you use CCK taxonomy field for a content type (in my case, bio module), the default will be the first term in the list. As a result, you can see a lot of fake users with languages "abkhazian" on my site. Any tips on this? Thanks!