Why do you think his ideas are morally wrong. In my experience, almost every disagreement that is perceived to be moral is actual factual. That is to say, we have a disagreement on what the consequences of certain actions will be. These disagreements could be deeply rooted in ideology (as in, what is the consequences of wealth inequality), but are factual, nor moral.

Further, by not engaging, you miss the opportunity to present why you think his ideas are wrong. If people hear both sides, they can make an informed choice. If they hear neither side, they can only make an uninformed choice. If you think this arrangement benefits you, I would question how confident you are in your beliefs.

The way to stop people like him without killing the foundation of our democracy is to explain rationally why his ideas are terrible and why no one should listen to them, not to prevent him from speaking at all. No one is suggesting that you do nothing.

Yes but in the long run, unless you are to be crowned the ultimate arbiter of what ideas are correct, the only effective way to do this is to persuade people (sometimes including the aforementioned immoral actor) that these actions are wrong. The alternative is constant policing of every action, instead of trying to shape the way decisions are made.