If a person blatantly stands in AoEs and makes me use a ton of mana to heal them, I don't heal them. Simple as that. There are others in the group that are working hard to get through the content.

Granted, there are quite a number of AoEs or other telegraphed attacks one can stand in and take minimal damage/debuff w/o greatly impacting DPS or finishing a boss off.

There is a line between the two, however.