[Harelin]

The forum spam problem is easily solved if you ask a question during registration like: "What are the last four letters in the name of this website?" Anything that requires momentary thought from an actual user, and is specific to your website, will throw bots off.

[nodja]

This only mitigates spam if your forum is small enough for spammers to not care. Those questions are a mere extra parameter on the registration post request. Spam software automatically detects those and asks a human to fill it. If the question is static it's even easier than a captcha solver.

[ryacko]

Spam is done by massive botnets that crawl the web at a rate nearly equal to Google's.

[AFNobody]

And they have their equivalents of Amazon's Mechanical TURK to learn how to bypass security questions.

https://2captcha.com/public_statistics <- this for example

You can find humans willing to work for $0.001/captcha.

[littlestymaar]

I've also read that some torrent or other illegal download website display captchas that are in fact captchas from other websites, to crowdsource the captcha bypass.

[AFNobody]

Correct. There is a long list of human farming options they employ.

They frequently use that one then "pretend" they have workers.

[AFNobody]

As someone who uses stuff like that to reduce spam, it cuts spam by about ~50%.

Many of them rely on human operator intervention for forum registration when the automated method(s) fail these days.