Something that one absolutely and positively cannot stand is anathema. Garlic is anathema to vampires (ditto for stakes and daylight). So is kryptonite to Superman or a silver bullet to a werewolf.
Originally the term anathema comes from the Catholic practice of denouncing a particular individual or idea that was antithetical to the Catholic Church. If done to a person, it excommunicated them, meaning they could no longer partake in the church's sacraments (with presumably pretty poor consequences for the soul.) That's a lot worse than kryptonite.