I'd like to know the same thing. In fact I've only seen like 2 paid relays and there doesn't seem to be a list anywhere.
nostr.wine seems to be the main one but they don't seem to advertise their prices and I'm not a member of them.
I tried some of the relay software and there doesn't seem to be an easy way to delete spam or see reports. The relay software is definitely lacking features for administrators.
In sure eventually we will seem improvements but charging micro transactions will definitely solve the problem.
I charge 0.10 cents to register on my forum and it completely got rid of spam. I could even go lower.
Stay hopeful