bullet said:
How easy or hard would it be to filter who needs to use captchas?
It isn't that easy, because people with the right intentions do the same thing as the spammer: posting a comment. The content of that comment differs.
Of course you could track each member and if he visits pages or puts comments too fast (when is that even?) you could detect the dumbest spambots. A smarter spambot makes intervals before navigating a page, like it's reading the content at human speed.
About the "How long are you registered here"-limit: What if a (very desperate) spammer registers an account now, but till 2 years he puts it in "sleeper"-mode and does nothing with it till the spam account is 2 years old. Limit defeated.
About the defeats of the captchas: I doubt a spambot solves them by itself. It could also "forward" them to other, more dubious sites where visitors solve that captcha to get whatever from that site, while they didn't know that they just helped a spambot. At least I've heard of such practice and I won't be suprised such method is applied for defeating comment captchas.
And yes, it's fucking slow. Given the content of the puzzle, I wonder even if we aren't a part of the so-called AI for Google's self-driving cars.
Furthermore, I won't be surprised if -apart from the spamming- the captchas were put on the comments to demotivate participation of [spoiler=debates]flamewars[/spoiler].