[Taqwus]

Well, you could presumably block all mages in the game from casting Breath of Winter by modding it to have ludicrous or impossible requirements such as 3100 fatigue, which also imposes 31 gem cost and thus more than a caster can carry. This wouldn't be terribly fair unless none of the nations in the game could make use of it, however, e.g. Niefelheim. You might also be able to make a spell unresearchable, don't recall.