If someone's action is closing the stable door after the horse has bolted, it is too late because the thing it is designed to prevent has already happened. It is nice to see the water company offering a reward to catch the fish killers, even though it might look like closing the stable door after the horse has bolted. People have been complaining about a strong smell of gas for seven years. This is closing the barn door after the horse has bolted. Note: This expression is often varied. For instance, you can use shut instead of close, and words such as gone or escaped instead of bolted. The children should have been vaccinated earlier. It seems like they are shutting the stable door after the horse has bolted. Note: People sometimes just say close the stable door or close the barn door. Journalists exposed the fraud, since when the regulators have been scuttling to close the stable door.