My idea on how to get rid of SPAM. Any takers?

    I've had this idea for a while and at one point in time was planning to implement it in the real world. However, due to time constraints, it never worked out.
    First, a little background information. A lot of web sites today use what has been coined CAPTCHA (Completely Automated Public Turing Test to Tell Computers and Humans Apart). The problem with this, however, is two-fold. First of all, most people are annoyed by having to enter the letters and numbers they see in a picture EVERY time they want to post a comment on a blog or a question on a message board. They're even more annoyed when the characters are scrambled beyond recognition, thus forcing them to either refresh the page or use another means of verification. The other problem is that CAPTCHAs are not 100% effective (see "social engineering").
    So, here's my simple idea on how we could get around this. What if, instead of having to prove that they are humans to each individual website they visit, people could prove that they are humans only one time (or, maybe, once per day)?
    How could this be implemented? One way is if there were a website that would allow people to sign-in, take the CAPTCHA test once, and would keep them signed-in through out their internet session. Other sites could then determine if the user was signed-in, thus proving that they are human. This would take care of the annoyance problem, but what about the social engineering problem? This could be handled by requiring users of the website to pay a small monthly or annual fee, just enough to keep it anti-cost-effective for spammers. It seems that everyone would benefit from this, because the verification website could focus on new and improved ways of verifying humans, and the other sites could focus on their intended goals instead of having to worry about spammers. Privacy would be the only negative, although there are ways that this could be addressed.
    So, there you have it in a nutshell. If I get a lot of feedback on this, I may post more specifically on how this could be implemented; but for now, I just wanted to get the general idea out there so that someone might be inspired to put it to use.