lock the barn door after the horse has bolted
Also, lock the stable door after the horse is stolen. Take precautions after damage has occurred. For example, After the burglary they installed an alarm system, but it's locking the barn door, or Deciding to negotiate now after they've been fired—that's a matter of locking the stable door after the horse is stolen. These expressions of action that is useless because it comes too late have long been proverbs in many languages and first appeared in English in the mid-1300s.