The Riot Act???
It’s a colloquial expression, someone in authority telling people to get in line and behave in no uncertain terms.
When you asked about it, I thought I’d look it up. I found this:
The Riot Act...”An act for preventing tumults and riotous assemblies, and for the more speedy and effectual punishing the rioters”, came into force on August 1, 1715, and remained on the statute books in the United Kingdom until its repeal in 1973.