For a permanent solution, it will to an extent require us to give up a level of anonymity. Whether it's linking a discussion with a real life meetup... like this (NSFW warning)
or some sort of government tracking system.
When nobody knows whether you are a dog posting on the internet, or a robot or a human, mitigations like Captcha and challenge questions only will slow AI down but can't hold it off forever.
It doesn't help that on Reddit (where there are/were a lot of interacting users), the top voted discussion is often the same tired memes/puns. That's a handicap to allow AI to better imitate human users.
Yes, this is the solution. Each user needs to know a certain critical number of other users in person who they can trust (and trust that they won't lie about bots, like u/spez) in order for there to be a mesh of trust where you can verify if any user is human in a max of 6 hops.
tl;dr: if you have no real-life friends...it's all bots :P
That sounds like the PGP Web of Trust, which has been in use for a long time and provides cryptographic signatures and encryption, particularly (but not only) for email.