Relays issuing their own ecash tokens in exchange for access
Aut-ct tokens
Proof of work (I don't favor this but it is especially simple)
To be clear we're talking about defending against high volume spam here; not against low quality or objectionable content.
quotingThe Price of Attention: Attaching Bitcoin Fees to Nostr Events
nevent1q…cdly
https://github.com/spesmilo/notary/blob/master/notary.pdf
