[Taqwus]

If you don't like Magic Duel, there's an official mod for it that makes it cost a gem (thus reducing the chance it'll be used), and you could block the spell entirely by making it unavailable. You could do the same to Breath of Winter, albeit that one can be used with no risk by some mages when combined with the right troops.
But yes, water mages can be dangerous; even if they start away from everyone else, there's no guarantee that they won't move closer to the rest of yours.