Verifiable credentials are all about this. You need some sort of credentialing body that generates the credential for you, but after that you'll just have an opaque identifier. Any caller that wants to verify whether you're human submits the id to a verifier and the verifier says yes or no. You can also do attestations like age, so gate a forum on 16+ or something. You never end up having to actually give away your name or any other details.
What happens when someone agrees to sell or give away their id? The credentialing body could catch the very worst abusers who seem to be signing in to various sites and services multiple times an hour, but would fail to catch anything else.
I don't think you'll ever be fully free of spam, so you'll still need to filter bad content. If credentials get sold and used to spam, they'll get banned.
How do you ban credentials if they're anonymous? Notice that if you can tell two requests are from the same person then you can do it across services by both of them pretending to be the same service.
Also, what happens to someone whose credentials are compromised? Are you going to ban the credentials of the victim rather than the perpetrator?