The Real Opponent of the Reciprocity Rule

How to say no when up against a requester who employs the rule for reciprocation? You and i face a formidable foe. Whether by presenting us with an initial favor or initial concession, the requester will have enlisted a powerful ally in the campaign for our compliance. If we are not to be abused by it, we must take steps to diffuse its energy. But how does one go about neutralizing the effect of a social rule like that for reciprocity?