You can mute or leave group message threads.
Part of a group iMessage thread that isn’t relevant to you? Is your iPhone blowing up with message that you couldn’t care less about? You can disable notifications for that thread by opening that thread, tapping on the “Details” button, and enabling the “Do Not Disturb” switch. You can also leave the iMessage conversation entirely. Tap on the “Details” button, and swipe right against your name in the members list to remove yourself from the conversation.