Banning doesn't work on Nostr except for the the extreme measure of the relay blocking the client IP. Since they often can't meaningfully block IP's without collateral damage and administrative burden, public relays can't do much to truly block a bad user as it is costless to generate more npubs.

"Ban" in your client is only hiding those messages. Your client still needs to download their garbage in order to filter it from your view. Maybe you don't notice but client-side filtering means both the relay and your own resources are being consumed by messages you don't want to see.

Some clients tell users to cope better with resource consumption by telling them to turn off Global and Public Chats entirely. Then you download a lot less since you see only people you follow.

The only real solution here is to use paid relays where there is a cost to getting blocked by the relay.

Web of Trust informing relay-to-relay synchronization of a "Strong Set" of popular npubs would go far to reduce spam while also helping the relay servers to cope with load.