A little advice from a guy learning to live my life in a wheelchair...
Don't take yourself so seriously. Life is tough and if you can't laugh at yourself a little.. you're going to be miserable.
In a "Danny's" situation, let them know that you see them.. crack a small joke, share a laugh. If they continue, take a few shots at them. Something like "you're never going to make assistant manager, if you keep picking on the customers"...