This could potentially become a NIP. There are multiple ways it could work, the easiest might be this: For individual posts, you choose who can reply to it. This will only be client enforced, but that's fine. Could be done with a simple tag, something like: "following-only". If that tag exists, then clients should disable the reply buttons if the original poster does not follow you. And it should filter out all events not posted by people you follow. I'm tempted to implement this already, because you are correct that getting the spam away is important - and this will work fully without users needing to enable some Web of Trust filter. Nostria allows me to set "Following" as a filter on any post, but that's my following, I will implement another option that will only show replies by those that poster follow, and disable reply options and reactions. This way, we can begin to test this behavior before actually changing how the events are constructed.