Don't lock the stable door after the horse has been stolen
It's foolish to take precautions after the damage they would have prevented has already been done. Another version of this saying is “Don't close the barn door after the horse runs away.”
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.