- ID/proof of human verification. Scan your ID, give me your phone number, rotate your head around while holding up a piece of paper etc. note that some sites already do this by proxy when they whitelist like 5 big email providers they accept for a new account.
- Going invite only. Self explanatory and works quite well to prevent spam, but limits growth. lobste.rs and private trackers come to mind as an example.
- Playing a whack-a-mole with spammers (and losing eventually). 4chan does this by requiring you to solve a captcha and requires you to pass the cloudflare turnstile that may or may not do some browser fingerprinting/bot detection. CF is probably pretty good at deanonimizing you through this process too.
All options sound pretty grim to me. Im not looking forward to the AI spam era of the internet.