How is it any different than someone spouting any other opinion? It's not an opinion I agree with,;personally I think holocaust deniers are ignorant, bigoted morons.
You rebut the speech; you don't silence the speaker. That slope is just too slippery.
We as a people never participated in the Holocaust as willing partners - Check the history of how many countries distanced themselves by stating - Not us - it was the Nazi's - Austria is just a recent addition to a long list acknowledging their active participation in the Holocaust. The Nazi's had lots of help in this. Check the history.
That changes peoples of those countries opinion - They I understand consider denial of the Holocaust a Hate Crime.
Still lots of free speech - demonstrations etc in Europe.