Close, lock, etc. the stable door after the horse has bolted
Definition 1 of 1
Pro Tip 1/3Late Response
Used when actions are taken too late to be effective. 
After the flood, they raised the bridge, like closing the stable door after the horse has bolted.
Pro Tip 2/3Pointless Action
Highlights an action that cannot undo past harm. 
He apologized after cheating, a classic case of closing the stable door after the horse has bolted.
Pro Tip 3/3Irony
Often used to point out the irony of a futile gesture. 
Adding more security after the theft felt like locking the stable door after the horse had bolted.