You still are oversimplifying it. I don't know if you are aware of some way to do this or if you are simply thinking, theoretically, that there "must be a way". If you know of a way, I'm happy to learn it, but as far as I know, WC3 does not have a spell, feature or glitch to turn all spell damage negative. Object Editor won't work, either, because only certain spells can heal.