I’m not sure paid relays are a suitable solution for spam mitigation. If I can pay $0.50-1.00 one time to be able to spam some call to action (like a referral or buy some product), the chance I can make 10x off that $0.50 one time expense is very high.

I could create 500+ rotating pubkeys with paid relay invoices and just post from a selection of 20-100 per day. In this case it’s the content you’d rather block/limit rather than the pubkeys themselves.

It may help however limit the new randomised pubkey spam posts with a floating pubkey.

Other low hanging fruit includes relays that still don’t rate limit enough. If I see the same note content posted by the same pubkey, or even different every 5-10 seconds, that’s certainly something that per pubkey and per IP should be throttled at the relay level. Even a check - has this pubkey posted the exact same message content more than N times the past 25 hours.