Well if they're being rowdy and it's annoying me when I ref I treat them as a mentor and book them. Generally does the job. It's actually a very good way to keep real mentors in check as well as they don't want to be forced away from the sideline so generally behave themselves after receiving a yellow.
Another good one (to keep for very special occasions) is something like this.
Four years ago I was refereeing a game out in Kiltipper (past Tallaght and a long long way from Dunlaoghaire where I live) between Thomas Davis and Fingallians under 15s in Gaelic football. Two mentors, one from each side, encroached on the pitch several times engaging in rather loud arguments with one another in the early stages. After about ten minutes of the game I decided that was enough. Upon their next encroachment I told them rather directly that "This is a football match, not the Jerry Springer show, now shut up and get off the pitch." The words 'back in your box' come to mind as they both kept their mouths shut for the rest of the game.
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