If someone believes the holocast didn't exist they are clearly bonkers, but does that mean they should be banned from articulating that view? Surely the best way to defeat such idiots is to expose their specious arguments? That's what you do in a free society.
It also sets a dangerous precedent. I can quite foresee people arguing in favour of banning so-called climate change denial at some point in the future. The phrase 'first they came' springs to mind. More in the FT HERE.