Maybe, I think even just having to provide your credit card details would be enough and Youtube already does something like that where in some countries you have to show ID to prove you are an adult in order to watch some content. That would work as well.

But the issue in both cases is that it takes time to do and so many people would be too lazy to do it(I never provided Youtube with my ID when they asked for that reason) but also there are many people who would not feel comfortable providing such ID. Though to be fair a lot of those people are the kind that a website would be better off without anyway.

I'm too tired to give much of a response right now, but I've said that this is something imageboards should look into before, here on kissu. We (kissu) are protected by obscurity, but it's still something I worry about in the future. The internet is full of spam so adding semi-coherent AI spam to it will greatly worsen things for those few places that still allow free, open communication without proof of identity. As it stands currently, humans are worse than AI for the "identify the thing" CAPTCHAs that were a mainstay of the internet for so long.

I think the meguca captcha centered on anime characters is probably something that will serve a greater purpose one day, but customized and perhaps with another layer of perplexity. I've seen CAPTCHAs that involve levels of reasoning that AI won't (yet) pick up on, like a "choose the fastest" and the correct answer was to select the horse icons. I've also seen isometric views where you need to rotate an object to set it into the right position. Perhaps a kissu-related one would be something like "drag the nekomimi hat onto the right character", but you don't want it to be too complex or niche because you still want a broad audience.

Due to the effort involved this would need to be something of a posting window unlock instead of just one post per captcha since it would be far too annoying otherwise.