Another spam-free-relay experiment: 

wss://relay.nostr.band/trusted

This endpoint (with /trusted path) has all the nostr data from all relays, but it hides all posts and profiles that have low trust rank, unless you specifically ask for them by id or pubkey. 

In case of a spam attack on the network you can remove 'read' permissions from all other relays (or click 'disconnect' if your client allows that), and leave 'read' for this relay, and have content from all the network visible, except for spam. When attack is over - re-activate read permissions or re-connect to other relays.

Trust rank is an internal metric similar to PageRank. The upside of it is that it's very robust against sybil attacks, i.e. bots that interact with themselves to pump their likes/follows/vanity metrics gain nothing since only interactions from broader network would count. 

The downside of using trust rank for spam-filtering is that new blank accounts that haven't had any interaction from trusted accounts will not be visible, so you won't be able to see and greet new nostr users. 

The relay is free for now. Please let me know what you think of this approach.