"leave this conversation" is grayed out, and I can't stop messages
I am getting harassed by dozens of hostile messages sent to my phone and email. There seems to be no way to block the messages, block the sender, or to leave the conversation. This has been going on weeks now. The Details > Leave this Conversation is grayed out. I am getting these messages both on my iphone (messages app) and the mac messages app.
iPhone 6 Plus, iOS 8.4