How about expanding the n-th degree follows to the relay, too? If the author isn't followed by anybody of the relay maintainer's n-th follows, reject their event. #[4] commented about ephemeral authors being bad. Yes, they are. They are indistiguishable from spammers.

I feel to some extent spam will have to be addressed by standard server-side rate limiting and banning of abusive IP addresses at the relay level, that at least helps keep the complexity out of the protocol itself