I am not sure if I understood your example correctly, but here is how it all works with FEP-171b / Conversation Containers:
- A follows B
- C comments on B's post (Create activity)
- B generates Add<Create> activity and sends it to A
- A receives Add and adds C's comment to their copy of the conversation
If C is banned for spam (and their comment removed):
- B removes C's comment (Delete activity)
- B generates Add<Delete> activity and sends it to A
- A receives activity and removes C's comment from the conversation
