This happened to me about a month ago. I was tanking a roulette, (I can't remember which dungeon, but it was fairly low level), and there was a DRG who kept running in front of me to pull. After the first time I asked them not to do it again. Then they did it a second time. Exasperated, I told them to please stop. When I saw them about to do it a third time I yelled in all bold "STOP!" They actually froze. I told them I would let them die if they attacked, and the healer backed me up. I told them to get behind me and stay there. And they did.