Easy, same way the experimental feature in element / adblocks do it. You create block lists people can subscribe to in their client regardless of which relay. Both blacklist and whitelist modes. That way people can tailor the experience tp their liking using lists they trust. If a list gets overly censorious you just replace it.
Discussion
it is very easy to automate the provision on new npubs to spam with so I don't see how blacklists at that level would work at scale
whitelists would work but be suboptimal imho
