If you don't like "accounts of well-known service providers" -- email or login-with-whoever then sources of identity that "everyone" has that are hard to get many of are government ids, phone numbers and credit cards.
Like what else is there? For super technical people we could do something like "proof of bitcoin address" where we verify control of a wallet with some minimum long-standing balance. We could make you submit a recording of yourself saying some random phrase and then do some facial recognition to detect duplicates. IP banning hasn't worked since the 90's.