Leave or Mute Group Conversation in the Messages App

In Messages, you can leave a group text at any time, as long as there are three other people on the thread. 

How to leave a group text

1. Go to the group text message that you want to leave.
2. Tap the top of the conversation. 
3. Tap the Info button, then tap Leave this Conversation.

You can also "hide" or mute alerts from the thread so that you aren't bothered by all of the notifications that pop up.