[close|shut|lock] the (stable|barn) door after the horse (has bolted|is stolen)

to try to prevent something bad from happening after it has already happened

The origin of the idiom "close the stable door after the horse has bolted" goes back to a time when horses were a valuable asset and were kept in stables or barns. If a horse were to escape, it would be difficult to catch, and it would be too late to try to prevent it from happening once the horse had already bolted or been stolen. Therefore, the idiom implies that it's better to take preventative measures beforehand rather than trying to fix a problem after it has already occurred.

After the company's sensitive information was leaked, they installed new security measures, but it was like closing the stable door after the horse had bolted.
The city council decided to increase funding for road repairs after a fatal accident had already taken place.It was like shutting the barn door after the horse is stolen.
The city council started taking measures to reduce crime in an area only after several incidents had already taken place.It was like shutting the stable door after the horse had bolted.
The government passed a law to regulate a harmful substance after it had already caused significant damage to public health.It was like locking the barn door after the horse is stolen.
The homeowner installed a security system after a break-in had already taken place.It was like locking the stable door after the horse had bolted.