If someone can say no, they do not need the Heimlich -- at worst, they have a partially occluded airway, in which case using a finger to try to sweep the object out, or turning the person face down so the object falls out, is the correct course of action. (of course, if someone is non-verbally communicating no, then what you say applies, but I'd still be getting into position, and gesturing either sweep the mouth themselves, or use a convenient chair to do it.)