I've encountered blogs where the article comments had out-of-context replies with links, and also boards with out-of-context texts in replies to topics being many years old; sometimes a random quote was used.
One old method is also to register but not even consider following the activation link: the account details exist already, and the username may be the product name or website domain already. Or using the activation and then adjusting the account details, so the website contact field is filled. Now those links/details will also be crawled by search engines, should the board's memberlist be accessible - that's all what was wanted.
One key to avoiding spammers of any kind is to modify the registration pages: different names for the elements, different layout, different workflow. Bots aren't flexible enough, and human spammers are also confused by this. Based on your perspective it also keeps you from idiots who fail to register due to being unable to even read.
I rather suspect a mixture, just like a download manager (i.e. JDownloader) which automates everything for you and you only need to solve each incoming CAPTCHA. Solving CAPTCHAs doesn't mean the human does everything else manually, too.