It got to the point where there was a debuff in arenas, but that's really not fun. Maybe you can think of a better option. I think big heals should be very expensive to use (high mana requirement). Making them take forever to cast doesn't work either, since there are too many stuns and interrupts. You'd never get a big heal off. It's very hard if not impossible to get a long cast off even with good set up and peeling.
Share your ideas of how to address the issue without making it no fun to heal.