Dear Whatever,
Would ask them why they thought that and go from there. Their statement alone would not stop me from offering Reiki, if that is what they wished. I would not suggest to them that Reiki might "break" a spell. My concern, if I did not know the person or had not Reikied them before and they appeared to have heightened fears about the "unseen" influence of others, is how Reiki would be perceived by them in relation to spells, power, energy, etc.
peace & joy,
holobon