The phrase "giving the go ahead" means to grant permission or approval. Its antonyms could include phrases such as "denial of permission," "withholding approval," or "rejection of the request." Other antonyms for this phrase might be "putting a halt to," "banishing," "prohibiting," or "refusing to authorize." Some more colloquial antonyms may include "nixing," "vetoing," or "shutting down." Using the appropriate antonym depends on the context of the situation and the tone of the conversation. Regardless, antonyms for "giving the go ahead" all ultimately indicate the lack of permission or approval being granted.