How hard would it be for Trump to say, "There's no place in this country for racists, bigots, Nazis, the KKK." That's the bare minimum.
If Trump had a little more guts, he'd go further: "Some of these vile people, unfortunately, support me and the Republican party. I'm here to tell them I don't want their support. I find their views repellent and I don't want them anywhere near me, or my party."