The word "give stamp of approval" means to indicate approval or permission to proceed with a particular action or decision. However, there are several antonyms for this phrase, including "withhold approval," "reject," "veto," "disapprove," and "prohibit." These words indicate a negative response to a proposal, plan or action. When someone withholds approval or disapproves, it implies that the proposed activity is not desirable or acceptable. Vetoing or prohibiting means that the proposed activity is not allowed, thus implicitly indicating disapproval. In summary, the opposite of "give stamp of approval give the go-ahead" implies rejection or a decision not to allow someone to proceed with a proposed action or plan.