Welcome to Android Central! I don't think you can leave a group text (not like you can with a Hangouts group chat). Basically, you have to ask everyone else to stop responding to that group text, and instead create a new group text without you.
There is no way for you to leave by your own actions.
One of the other users needs to start a new conversation with a new group that does not have you in the group.
Depending on which texting app you are using, there may be an option to put the conversation on "ignore" so that new messages into that conversation do not trigger the notification system.