We had some posts about this a while back. Ironically, the captcha doesn't really work that well because I've learned that some groups in some of those asian countries, actually pay people to sit and crack captchas, then they build a database of answers and thus can create a bot to get by them.
This info is copied from an older post.
First line of defense that I've figured out, is thankfully they all register from common email address. So in your Admin area, under Edit, click "Edit Bad Emails" and enter the following in there:
I also found some information on what to put in an htaccess file if you wish to block all China (and a few other suspicious countries) from your site all together. The info can be found here:
With that last link about China, if your site doesn't need visitors from China (ie if your site is in english, it may not need to receive that traffic. Thus using an htaccess file through your web host is the best way to block the bad traffic. We block those IP ranges here and it's helped a great deal.
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