Here's an idea for relay operators, particularly for communities: you could create a public relay that prevents spam by allowing people to post only after they have a certain amount of activity. If they're not AUTHing, or REQing, they're likely spam. Ask them to engage more with existing content before posting.
are there any community-centric clients requiring an x-amount zap to join a specific group and simply using sats as spam prevention?
I think there are a lot things in the works like that, from nostr:nprofile1qythwumn8ghj7un9d3shjtnwdaehgu3wvfskuep0qydhwumn8ghj7argv4nx7un9wd6zumn0wd68yvfwvdhk6tcpzemhxue69uhku6t9dshxummnw3erztnrdakj7qg3waehxw309ahx7um5wgh8w6twv5hsz9thwden5te0wfjkccte9ejxzmt4wvhxjme0qqs2js6wu9j76qdjs6lvlsnhrmchqhf4xlg9rvu89zyf3nqq6hygt0svghc32 at least
This sounds trivially game-able