Go to your recent activity and click on the players name. You can then report them for griefing. I dunno if the reports do much but 🤷🏻‍♂️. “The solution to prevent this from happening is to only make your own fireteam. If everyone just made their own team and never joined one, these things would stop.” - the forums

Once boss encounter starts, the ability to kick should become nullified. This allows for legit leaders to kick people if need be after a failed run.