The phrase "give stamp approval" means to officially endorse or approve of something. Antonyms for this phrase include words like disapprove, reject, condemn, or deny. These words indicate a lack of support or approval for a certain action, decision, or proposal. While approval is often seen as a positive thing, it's important to consider both sides of an issue and make informed decisions. Sometimes, rejecting or denying something can lead to positive outcomes in the long run. Therefore, it's essential to have options and language to express disapproval or rejection when necessary.