Today I completely solved nostr's spam problem. The link below is a demo web client and relay you can try out — a pay-per-reply concept where posting a comment costs you 1 sat.
https://npub1pt4qwkcpp76et45pfatrxqywsuxh0gky84xa592yapesuqye29zql4eera.nsite.lol/This page is a plain nostr client (it follows the outbox model properly) that shows the 5 most recent notes from one hardcoded account. That account has exactly one inbox relay, and that relay charges.
Connect NWC and you can actually leave a comment, paying 1 sat for it. No login — comments are signed with a throwaway key. It uses ecash, so you need to top up 100 sats to start, but once you're done testing you can sweep the whole balance back to your own lightning address (2 sat fee).
If NWC supported keysend, no top-up would be needed and this could be genuinely pay-per-reply. But it doesn't, so there's no one-way push payment — doing it over lightning would take two round trips, which is why it's ecash instead.
I’m not a professional relay operator, and the relay isn’t optimized, so the connection may be somewhat unstable. Thank you for your understanding.