the horse has (already) left the barn

It is too late to prevent, change, or rectify some problem or situation, as the ill effects have already been wrought. Likely derived from the phrase "close the barn door after the horse has bolted." We're just now understanding how damaging these products can be for consumers, but considering they're already in the hands of millions of people, it seems that the horse has already left the barn. A: "I came to apologize. I never wanted to hurt your feelings." B: "Yes, well, that horse has left the barn, I'm afraid." My father quit smoking after he was diagnosed with lung cancer, so the horse had already left the barn by that point.