Spam is a huge problem. I easily get a couple of hundred spam emails in a day. It is also a problem on websites that accept user input, such as this one. There is nothing sadder than a guestbook or comment section that is overrun with spam. The reason that this happens is that one poor human with a real life cannot compete with millions of spam producing robots. It gets worse when the site becomes a group site. After a while, you get kinda of a knack for spotting bogus registrations. ALLCAPS is a clue, that one is a real human. Usually it takes a look at the email address to sort the miscreants and the n'er-do-wells.

This sort of thing takes some time. Personally, I like it. It gives me pleasure to squash spammers like they was bugs. Problem is, it is not real-time.

I want my users to experience instant acceptance.

It is looking like some sort of reputation system may be the answer. Question is, how much are you willing to submit to a third party in order for this to work? Are you willing to have a single online identity?

As an administrator, I have to know this. Or guess.