imessages

Hi, someone added me in an imessages group chat and I have no way of leaving. When I click on the group icon there is no "leave group conversation". I do believe there are people who don't have iPhones in the group because messages are green. Is there any way I can leave?? If I delete messages, they still pop up when new ones are sent. Please help! I don't want those messages! Even if I silence/mute the messages they still pop up on imessages