My relays have become over-run with spam, and I don't have to time to figure out how to thwart it. As such, I am going to throw in the towel for now until the brighter minds figure out an easier way to deal with the problem.
Discussion
Why don't you just whitelist the npubs you want posting there. Other option is nip-05 domain whitelisting. I run 2 relays, one for private notes with my npub only whitelisted, second for my company with nip-05 domain. Using https://github.com/scsibug/nostr-rs-relay