The phrase "casts a spell on" typically suggests enchantment or bewitchment, but there are a number of antonyms to convey different meanings. One option might be "disenchant," which implies breaking the spell or dispelling the magic. Another possible antonym is "repel," suggesting a strong aversion or distaste for the subject in question. "Apathy" could also be considered an antonym, as it suggests a lack of emotional attachment or interest. Alternatively, one could use "rationally persuade" to indicate that convincing someone of something relies on logic and facts rather than mysticism. Ultimately, the choice of antonym will depend on the context in which the phrase is being used, and the precise meaning intended.