Not much, since nostr is designed to combat censorship the only thing we can do at best is report notes as span/scam and block, but it is not a global solution, the only solution would be what you say, have your own relay and only show the notes in your relay. It also occurs to me to mitigate a little spam is in nostri.chat hide the button ephemeral keys so it is a little more complicated. Also maybe add pow to make spam more expensive 👀.