FWIW my FeoBlog implementation dealt with both by only accepting content signed by people who were explicitly allowed on that server, or followed by one of those people.
Are there Nostr relay implementations that can do something similar?
Discussion
Check out https://relay.tools
There are paid relays, you can also have private relays I thing with whitelisted npubs. Relays have many policies for spam, requests by time frame etc, by ip address etc, but the most important is at user level, if you don't follow that npub you won't see much spam.