Ryios just described exactly how that encounter happens in nearly every pug. I personally let it eat the black mages face. Then I explain what happened, and how to avoid it happening again. If it happens again, I just let it eat his face again. He will either learn, or drop group. I get instant queues, it's not really hurting me too much.

But ya, a 3 second force taunt would be handy when said black mages takes off at full sprint away from the tank to get away from the mob he ticked off. =P