I guess a nip could be created to attach a block-list of npubs to one’s own account. Nostr clients would have to adopt the standard and not display the block-list of the original note author. This would give everyone the choice if they want to display blocked comments or not. As a user I personally don’t want to see spam replies in most cases and am grateful if a note author is maintaining a spam-filter for their replies. If I’m curious anyway to see a particular blocked reply anyway, I could always opt-in to see it.