Don't be discouraged. Many Ppl do that in the Low Level because they now the mobs can't harm them. Let em do it. They stop doing it in High Level and if not, they can simply keep it. Tell the heal to stop healin them if he won't: his share of more work.

Just mark the mobs with 1 2 3 4 and if someone thinks he MUST pull aggro of the 3 or 4. he can keep it. Your Job is to ensure that the heal stays alive cuz he let's you survive. Don't give a damn about the DPS. If they don't play after your rules they can die for all you care. Simple egoistical rules like that make the tanking way more relaxed. And the DPS will learn it after a while.

If not they just die through the whole dungeon. Not my repair cost bill.