Naive Bayes spam classifiers are fast enough that I think they could be used as drop-in replacements, and they'd be well-trained too considering Google's existing success with spam in email. And Naive Bayes isn't the only solution, there are plenty of other heuristics. Even something as dumb as "first post from this user/IP/'identity' and full of links?" would catch a lot of common spam.
I really wish HN required a comment with a downvote on accounts with more than 100 karma (where one can reasonably assume the user isn't a newbie leaving empty comments like "I agree"). I'd ask that person to think it through: why doesn't gmail require a captcha every time you want to send an email? What about other mail providers? What about your own native client? An internet without working captchhas[sic] will probably be full of bots and spam. Is your inbox full of bots and spam? Mine isn't. Even my spam box is at less than 800 over the period of a month; before one of the big botnets was taken down a few years ago that was generating most of the world's spam I still had less than 4000 over a month.
