If a chat is started but then one party decides they don't want further replies/chats, there is no (obvious) way to prevent the other person from re-opening the DM exchange if that other person hasn't left the chat. Blocking does work but that affects whole accounts & not very elegant.

yes we are considering something like "dm blocks"

intuitively it should just inherit the "no one" setting irrespective of previous chats imo. The presumption is the "no one" setting allows outbound initiations only, then leaving chat closes that inbound interaction.