"The solution to this is extremely simple. Simply add a question to the registration page that only a human with knowledge of the site can answer..."
Yeah, these are all good delaying tacticts. AI's have time. They can do anything you can think of. And this is another good solution.
AI has many definitions, but I like to think of them as Turing did, as a response generator whose responses cannot be distinguished, person and machine. After all, that IS what we are talking about. How to distinquish between man and machine?
Every potential new member could be a robot. How do you distinquish them apart?
You can not. Someone will build a better AI and log in, eventually.
Only by checking a potential new member's intent to use his membership can we check. Like using a different algorithim to solve complex multiplication (used for encryption,) checking a persons "target of intent" becomes a matter of not validating his signing up (the obvious "become a member") but checking his posts for some valid content ("I have a question about sleeping bags...")
IFF the AI can solve this, then it doesn't matter, his posts are "valid" within the context of this site. I don't care if he is a person or machine. His posts are fine. He might start spamming after ten or twenty posts, but even people's computers have been known to be hacked and start spamming.
IFF his first post is spam, well, he needs to be gotten rid of(I favor hanging by his protruding member rendering him impotent...metaphorically speaking, of corse.)
So, I would suggest a "logic" check by simply moderating his first couple posts rather than an "item" check that can be solved by brute force.
But, there is never a guarentee that a bot will not figure out something general in response, like "This is a great site!" Sorry, it needs to show a specific intent. Something that is NOT gleaned from anything on the web page, and by extension, the web site. And not so general that it can be responded to by simple word substitution for phrases on the page. "I like backpacking" from the web site would not be good enough. "I like" is general, "backpacking" was gleaned from the web page.
I think Roger is Australia's AI. How do I know? Maybe I am an AI. How do you know? Does it matter as long as Roger stays on subject, there, and I stay on the subject, here?