I can think of two ideas, neither one perfect, but might be of use nevertheless.

Idea 1: a relay can agree never to send content from userA to userB if userA blocks userB. This could perhaps be a paid feature of specialized relays. Things like notes getting leaked to blastr would present challenges but we might be able to address them.

Idea 2, more complicated but more effective: put all content onto a single, specialized personal relay. Your notes only get broadcast to whitelisted accounts. You could even encrypt each note, similar to a DM, to mitigate the occurrence of notes getting leaked to a blastr-like relay.