Blow the whistle

1.  Expose corruption or other wrongdoing, as in The President's speech blew the whistle on the opposition's leaking information. [Colloquial; 1930s]
2.  Put a stop to, as in The registry decided to blow the whistle on new vanity plates. The term originally alluded to ending an activity (such as factory work) with the blast of a whistle. [Late 1800s]