Pay-to-follow would basically prevent everyone from getting their first followers. That's clearly too extreme. Pay-to-comment seems impossible to enforce, because there isn't such a thing as 'comments turned off' in NOSTR anyway.

Pay-to-relay is the one I'm hearing most often but it's not enough IMHO. If a spammer just paid 1 relay with a large reach it would still get to tons of people. There would always be free relays too, anyway.

I'm thinking a real attempt to design spam blockers here would be another 2nd layer technology, perhaps a paid whitelist built into clients, or some kind of web of trust.