Usually the spammers are bots, but every once in a while, it is actually a person. Spam bots crawl the web looking for websites, register, and post spam. Spam bots process data, they do not think. This is why they beat captcha image verification.
How do you beat the spammers? You have to be proactive, and not reactive. I got proactive on my sites. I have not had a spam bot on any of my sites in over 4 months! I have measures(different from captcha) in place that prevent them from registering. Keep the door closed so they can not register, and you beat the spammers!
I do not post my solutions on what I do to prevent them in forums, because I believe there are programmers who read these forums and reprogram the bots to beat the fixes. I will share how I beat them in a private message or on msn, as long as the information I share does not get posted on any other site!
Mike