The phrase "gives stamp approval" suggests an affirmation of something or someone, indicating approval or validation. However, there are several antonyms for this phrase that imply rejection or disapproval. Words such as disallow, veto, or decline all indicate a negative response, dismissing the subject in question. Other options could be words such as negate or nullify, which suggest a more complete dismissal or cancellation of something. While giving a stamp of approval is often viewed as a positive thing, it's important to remember that negative responses can also provide valuable feedback and help to refine or refocus efforts.