This is a situation that all umpire's at some point and at some level will encounter. Some my disagree, but it has been my experience that I concentrate on everything that has to do with the players and coaches on the field and the dugouts. As an umpire, we are responsible for managing the game within the fence. Once you start listening to fans and take things personal then you are not 100% focused on doing your job.
Saying that, there should always be a school administrater, tournament director or a point of contact that should handle any problem fans in the stands. When I arrive at a game site the first thing that I ask is who is in charge of administration duties and find that person, introduce myself and go over any possible situations that could come up and let him know that i will be in charge of everything on the field and I will expect him/her to handle any situations that may arise outside the field that might cause any disruption of the game.
When you do that, you are doing preventive umpiring that will allow you to be prepared should a situation come up. When you have a plan of action for things like this, you will see it nipped in the bud and I promise you that the game administrater will appreciate your attention to detail and including he/she in your game plan.
I would definitly let the catcher know, in a discrete way, that if he is going to acknowledge the heckler and converse with him that he will not be in the game long for the simple fact that he is a player on the field.
It is hard to listen to stupid people that don't know anything about the game, but you as an umpire have enough to take care of on the field so have thick skin and focus on your task at hand. It took me a long time to learn that but after I did, it was a load taken off of me and I became more in tuned and sharper at my job as an umpire.