> You cannot argue against people that believe an entire race or an entire people should be systematically eradicated.
I think we can, and indeed should argue against such ideas. The goal is not to convince a zealot that his views are wrong, but rather convince the majority of those on the sidelines that those ideas are stupid and will lead to no good. If the ideas are outlandish, arguing against them on merits is easier, not harder. As long as the vast majority of the society sees those ideas as crazy, arguing them stays pretty harmless.
And what is the alternative to arguing on merits? It is tempting to be able to silence unpopular views, but that can backfire -- tomorrow control could change and someone might deny us the ability to speak. "I know the world is flat" needs a better answer than "No! It is round, end of discussion".