Refuse to take him seriously.
Seinfeld- recognizing that when all is said and done Trump is merely a heckler with a lot of money and a very large audience who deals with his own insecurities by treating others in a juvenile fashion- the same way he'd treat any heckler of his nightclub act.
People react to The Donald in pretty much one of two ways. It seems from the polls that lots of people treat his childish behavior as a way of venting their own hostility and frustration with illegal immigration and pretty much any other subject Trump decides to demagogue on. The worst thing that we could do is to validate their hostility- not necessarily their concerns, but their irrational hostility- by taking Trump's seriously.
The other way people react is with frustration and anger of their own. I fall into that category. We think, "This guy is so obviously irresponsible and immature and just plain silly that it's outrageous that so many people not only take him seriously, but buy into his act!" Perhaps we comfort ourselves with the knowledge that, when all is said and done, the American people are not going to elect this bozo president. In fact, when it actually comes down to it (at the latest, after the Iowa crazies and others early in the process have had the chance to vent, but probably before even that), rank-and-file Republicans will balk at nominating a man who engages in behavior we wouldn't tolerate from our toddlers. Certainly they're not going to throw away a golden opportunity to elect a conservative president in the nick of time- while there's still an outside chance of undoing the damage Barack Obama has done to the country, to rescue our role in the world at large, and perhaps even to turn around the Supreme Court and stage a counter-offensive in the Culture Wars- just to throw a tantrum by voting for an unelectable clown like Trump.
But as Seinfeld points out, to react to Trump with anger or frustration is to take him seriously- and to take him seriously is to validate him and encourage his fellow tantrum-throwers. The country can't afford that.
Better to take Jerry's advice and treat Trump like the heckler he is. Don't get mad at him. Express concern for the hangups and insecurities which cause Trump to act in such an inappropriate fashion- and no matter how he reacts (probably by going ballistic at first, and then by sulking and finally shutting up and going away), insist on focusing on Trump's personal insecurities rather than on the nutty behavior and outrageous statements with which he gives vent to him.
Treat him like what he is, not what he'd like us to think of him as being.
Here's hoping that somebody- maybe even everybody- takes Jerry's advice at the next debate. Donald Trump has already wasted too much of our time with his juvenile antics.