To leave a group chat, open the chat you would like to leave and tap the header to open Group Info. Scroll to the bottom of subgroup info and tap "Leave this subgroup."
If you don't want to leave, you can also Mute the group. To mute a specific group chat, choose mute in Group Info. You can choose to mute the chat for a choice of 8 hours, 1 week, or 1 year.