I don't have a solution to the problem and I concur it is a difficult problem to solve. A sibling comment has pointed towards
https://www.gov.uk/service-manual/technology/using-captchas, which doesn't have much enlightenment either.
That said, I'm merely pointing out the problems I see in the current "solution":
1. It is a very robotic approach to design a system: we end up treating human beings like numbers and thereby marginalizing the minority. After inconveniencing these people, we turn around and tell them: "just be normal" or "no you shouldn't think like this". If that's the type of society we would like to build, I think I have nothing else to say.
2. ultimately externalizes the cost of abuse detection to ads, which makes it very tempting for everyone to adopt it as it is "free". Even if differently motivated entity solved this problem, I'm unsure if anyone would want to pay for it given how invisible the issues it causes are to the business (as you have pointed out). This may mean that what I pointed out in (1)_will occur everywhere and I don't even think there's really a way to stop it from getting to the possible dystopia I see.
Maybe I'm just over dramatizing this and I really hope that is the case.