This is from the nostr GitHub.

Spam

If spam is a concern for a relay, it can require payment for publication or some other form of authentication, such as an email address or phone, and associate these internally with a pubkey that then gets to publish to that relay — or other anti-spam techniques, like hashcash or captchas. If a relay is being used as a spam vector, it can easily be unlisted by clients, which can continue to fetch updates from other relays.