I feel your pain. I think the only way to do it might be to avoid the Cataclysm, by not getting either Heloise or Valzira pregnant so that Malice is never born. But that would wipe out a huge chunk of the plot, and might even turn out to make it impossible to progress to Act V or at least successfully get to the end the game when it's eventually all been written, so would probably be too high a price to pay. (Well, I suppose the other way to do it would be to lose her from your game altogether prior to at V by giving her to your allies to increase their morale, but you clearly wouldn't want to do that.)