(Not suggesting this over captcha)

1. Ask questions for a personalized answer like you get on Lemmy sign ups. A human will have to verify each request, or.... you can put an AI bot to judge whether your response is botlike or human.
2. Require you to sign in and verify yourself with a valid identity
3. Require you to sign in and pay to use whatever

All options I can think of without captcha are either unscalable or require you to give up privacy.