Print these letters out on card stock, cut them out, and laminate for durability. Add magnetic tape or velcro dots in order to attach each letter to the board or wall. At the beginning of each day, put all five letters to spell noise on the board. If students get too noisy a letter will get taken away. Once all the letters are gone, no talking is allowed until the next day.
My students love this way of keeping the class noise level down. They remind me each day to put the letters back up, and when a letter gets taken down they encourage everyone around them to quiet down.