If Leave This Conversation Is Grayed Out
If Leave this Conversation is grayed out on your iPhone, there are only three participants in the iMessage conversation, and you can’t convert a three-person iMessage conversation to an iMessage between two people.
You can set the conversation to Do Not Disturb so that you aren't notified of new messages and just basically avoid reading it, perhaps even deleting it every once in a while if new messages come in.
I'm still missing something. Don't read the messages. It's not like anything person (A) does can stop person (B) from sending them a text message, other than blocking the contact.
I'm still missing something. Don't read the messages. It's not like anything person (A) does can stop person (B) from sending them a text message, other than blocking the contact.