There have been a number of threads for dealing with this situation. (My favorite response -- abeit one out of your control -- is for the home plate umpire when he sees this baserunning nonsense commense to call time after the ball reaches the catcher and 'inspect the ball', preventing the runner at third from advancing. When the offense's manager complains that you can't call time while the ball is live, the umpire can tell him that he'll stop doing it as soon as his team starts playing real baseball.) What you can do is to have the third baseman come half way to home plate after each pitch; if the runner dances off, the catcher can throw to him and have the third baseman walk the runner back to the bag and then throw to the pitcher. After awhile, the fan response if nothing else will force the offense to stop it. Also, judicious use of pump fakes may trap a few kids.