My only method was to keep yelling at the idiots sending messages and telling them to start a new group text without me. When they didn't do that, I just blocked all of them. (It was an accident--not people I know.)
SO, while that is not helpful, let me know if you find a solution.
I don't know of any way to do this with the stock app, but with Textra you can silence a thread, meaning you will get no notification for any new messages.