Ok pro dm tip it works every time: get a box with a thin bottom or a cookie tin, and a metal d20. Whenever you want the to shut up roll that d20 on that surface and after looking at it make steady, certain eye contact with one player, preferably the loudest.
I have never seen a room of grown men shut the f up so fast in my life. I made a man a full 30 years older than me and foot taller quail and stop making any sound. It works.
I am leaning toward blue, but which shade?