I would have done exactly that. You asked DD to attack marked target. Still wasn't happening through a few pulls. I would have announced at that point "If you aren't attacking the right target and pull aggro I will not heal you.". Then I would have proceeded to let that happen. Most of the lower level dungeons can be 3 manned anyways . If he continues now to try and disrupt the party in spite of not receiving heals a vote kick would be the next step.

Not passive aggressive because I state my problems and intentions and then consequences of not properly adhering. You need to treat some of these folks like kids because as I have found out a lot of them are/ or still have that mentality.