1. The boss had noticed that the employees frequently took more time than they were allowed for lunch and coffee breaks. The boss knew he had to put a stop to it, so he called a meeting and laid down the law
2. The teacher decided that he would no longer tolerate late homework, coming late to class, or chatting during class. When the students were all in their seats, he laid down the law

Synonyms: put (one ’s) foot down. Read (someone) the riot act implies more noisy anger against a past action than lay down the law , which implies stern instruction governing future behavior.