"I don't want npubX to be able to see my posts on #Nostr, how do I block them?"
You are posting unencrypted notes to public relays. How do you imagine that is even possible?
Your options are to post encrypted notes that only those with the decryption key can read, or only post to private relays that only those on a white-list can access and which respect the "-" tag to prevent rebroadcasting out to public relays.