I also don't see how bitcoin could be used as a captcha, those are used to make sure there's a human on the other end and not a spam bot...you could easily automate the payments. For this niche, it's paying for high bandwidth, concurrent downloads of pirated content; it just won't work otherwise IMO.
Bitcoin can't prove that someone is human, but requiring a very small payment for each "dangerous" action is perfect for preventing abuse.
For spammers that make thousands upon thousands of dollars a year? The payments would have to be higher than the potential profits of the spam and it would likely push it out of reach of the average consumer. I'm not going to pay $0.50 to post a blog comment, ever. A big spammer could do that a thousand times and still not cut into their bottom line. I think the fact that spammers use Mechanical Turk to break captchas already is the proof in the pudding.