all forms of spam prevention boil down to paid access, really
bitcoin uses proof of work, and uniquely can use it because it is money, and there is only one money, and the vast graveyards of proof of work shitcoins stands testament to the there can be only one problem
proof of work is to spam-prevent the PROVIDERS of the bitcoin service
the users have to pay transaction fees, which is the spam prevention
so the same applies to nostr, except that we don't have a consensus so there is no competition for that, we just have a user spam problem
so, we just need transaction fees, and because it's a relay, and it's not just one-off service, you switch to subscription model, but the problem is
you need auth to do paid subscription model!