From UmpYouthFP

Make the troublesome player the coach's problem, not yours. If you have a player or two on your case about the strike zone, or some player keeps whining about every call you've made on the bases so far, don't take it upon yourself to stop it. Throw it into the coach's lap. Don't wave your finger at a coach from sixty feet away demanding he do something with his player. Wait for the right moment, preferably between innings when nobody else is really paying attention. Sidle up to the coach and mention in a friendly conversational way that you're a bit tired of listening to his player complain about "everything." Chances are, the coach is tired of it as well and you're now giving him the opportunity to solve the problem. Let the coach know that if they're not able to do something about it, you may have to. You do not need to get specific, the coach will figure out what you're implying. Generally, this will work and you'll have no more trouble. There's nothing like peer pressure to solve a problem. If this doesn't work you'll have to do whatever is necessary. You have given both the coach and the troublesome player their chance. If you do have to eject the troublesome player, let the coach do it for you. That is, simply call the coach over and ask him for the name of the player who is now going to be playing and batting for the ejected player. When the coach gives you that confused look, quietly inform the coach that you need to know who is the ejected player's replacement. You do not need to get animated about it.