That's why I used to LOVE healing in FFXI. If some guy pissed you off, it was easy to let them die and still win the battle if you were good at the game.
Most of the time it happened, it was people going in trying to show they could just do tons of damage, but a big part of that game was aggro control. If someone let loose, they'd probably get the mob on them and then the healer has to work harder than needed.
I'd warn them once or maybe twice, the next time I'd let them die. It was a big deal because you lose XP if you die in that game, or at least you used to.