Of course is the spam not human. The human only solves the captcha and that's it. You can by 1000 captcha solutions for 50cent in china or other asian countries as it's stated in the wikipedia article. The bot does the registration up to the point where the captcha comes in place. Then a human solves the captcha and the bot continues its work. So *no* captcha can stop this, because the task of the captcha is to identify humans and humans are the one who solve them.