To remove yourself from a group message on an iPhone, go to details in the upper right corner. Scroll down. In red, there should be an option to Leave this Conversation.
Hope this helped!!!!
P.S.
People can unfortunately add you back into a group message.
:(
If you're using iMessenger on your iPhone and want to leave a group message that you've been invited to, then all you need to do is:
It's really that simple!
If you want to get rid of a one-on-one conversation, then just swipe to the left on the conversation preview in the "Messages" screen (the view that shows you all your conversations), and press delete when the red button slides into view.
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