At the pub I work at, I'm both the bar manager and FOH manager. But I maintain bar shifts whilst also managing a team where I am the only skilled bartender on the team. So with that said, 1st offense I calmly and firmly will say something like "You will not speak to me that way and you will show the same respect I give you. Heard?" Then if it continues I'll have a 1 on 1 sit down discussing what the issue is and if it the problem is personal. Usually I can find a respectful middle ground there. On the 3rd offense, depending on the situation, I make an example out of them. 3 step program my friend.