Although it's very offensive when people deny that the Holocaust ever happened, there's no way to truly ban this type of sentiment. Freedom of Speech allows people to state just about whatever they want, and people have denied all sorts of things from happening, such as saying that no one ever stepped foot on the moon. In order to protect people's freedom of speech, sadly we also have to let people make statements that we know are not true.
In my opinion, Holocaust denial cannot be appropriately defined so as to outlaw it in the United States. Because of our rights defending free speech. Our laws grant all men the right to say terrible thing. Our laws also defend that persons right to make those statements without fear of imprisonment or reprisal. To change that we would all have to give up that right being that we are all equal under the law.