I think we should ban all discussion on these topics:
- Jet fuel, jews etc
- The holocaust
- Whether fascism is leftwing or not
I have never seen a good discussion and they always go off the rails.
First of all, discussion is already sort-of banned; see What types of questions should I avoid asking? .
Second, questions about these topics are rightfully held to a higher standard on Stack Exchange. They must demonstrate some research in the topic and be (however hard that is to define) of 'academic' interest. We, all humans in this world, can't deny our collective history and should be able to talk about it in a neutral way that doesn't offend people. Only by learning from past mistakes we can avoid similar mistakes in the future.
You mention topics, like the holocoust, that must remain debatable in a democracy. And StackExchange is lagerly a democratic network. This may sound a little bit dramatic, but I would go so far that even if all resources of moderation are exhausted, it must be ensured that such topics can be discussed.