Antonyms for the phrase "gives the go-ahead" include words like deny, reject, decline, veto, and refuse. All of these words imply a negation or rejection of permission or approval. If someone denies or rejects a go-ahead, they might be indicating that they don't think it's a good idea, or they may have concerns about the plan or project. Alternatively, they could simply feel that the timing isn't right, or that other priorities take precedence. Regardless of the reason behind the refusal, antonyms for giving the go-ahead can signal a potential roadblock or drawback to proposed actions or initiatives.