You can ‘block’ people by being on your own relay that no one else has access to.

Aside from that, I don’t think this is possible because the blocked would need to ‘opt-in’ to their own blockage. Or the Nostr client would need to block the user on your behalf, but would have no effect if the blocked went to a different client.

Sorry. You’re gonna need to just not worry about it and just concern yourself with the things you have control over.