One time payment(1-5 usd) for relay whitelisting should be enough, they can pay, but once they get banned from the relay doubt they would pay again.
Truth, but what % of newcomers have a Ln wallet already to pay from?
They can still begin in open/free relays and with time pay for these relays, it's simpler that all other workarounds like Wot+ip + pow. These can still be used mainly by clients but paying one time it's the simplest and most effective way to fight spammers.
All the issues here are talking about spam on open/free relays. We have paid relay support in most clients.
One thing missing in damus is a mode that filters notes from non-paid 🤔 we only have that in global atm