That is difficult to do now. Since things like blastr send to all relays on nostr watch. So might have to block them by IP. But can't block blastr without blocking all of Cloudflare. Have discussed this on strfry Telegram. Needs a NIP seems. Like allow lists for relays as do with npubs.
Discussion
Definitely something I'm keeping an eye out for! Private non-rebroadcasting relays, plus clients that allow relay selection per note, or purpose built clients for communities is how we can replace things like closed telegram groups.