The only way I can see that working is by locking yourself away on a private relay that only people you choose to allow access to can see and then not posting to other relays where the person(s) you want blocked post.

I wonder, in fact, if it's feasible to have private members only relays where you post private content...?

But then, anyone interacting can just rebroadcast the note to other relays, if I understand nostr properly. 

Instead of a technical solution, which seems to me unfeasible, perhaps the answer is cultural change?