Sadly, I can't offer you a true solution aside from letting them rush off and inevitably die. If they're in that much of a hurry to tank things and get killed, so be it. I understand this isn't entirely in your control even as a tank, because it's your healer that may try to keep those nasty ninjas alive. Still, it's worth discussing with the rest of your party in the event you run into another one of them.

That all said, I've encountered innumerable DPS and healers that act similarly. Especially if they outgear the content. I may never have played a tank, but I do have a healer and DPS and I just feel behaving that way is both rude and unnecessary. I feel this disrespect is partly to blame for the severe lack of tanks being played these days. You don't want to have to put up with that nonsense, so why would you?