There are a variety of antonyms for the phrase "gave stamp approval." Some possible antonyms include rejection, disapproval, disavowal, and denunciation. When someone "gives stamp approval," they are essentially endorsing or supporting something. By contrast, rejecting something means that one refuses to accept or approve of it. Disapproval is a more general term that indicates a negative assessment or evaluation of something. Disavowal suggests a more active disowning or repudiation of previously held beliefs or actions. Finally, denunciation is a particularly strong and emphatic form of criticism that implies condemnation or censure.