Which ideas are you talking about?

Communism and National Socialism are two examples of what I had in mind for the 20th century.

If you are talking about something like communism, it wasn't really ignored.

That is certainly true. If we could somehow get everyone to simply ignore bad ideas, then yes, bad ideas would wither. The problem is, I have direct control over only what I ignore. Even if I choose to ignore a bad idea, others likely will not. Now lets suppose an entire group of really thoughtful people (e.g. the LW community, perhaps) could be convinced to ignore bad ideas - there are still going to be plenty of people discussing bad ideas, and so bad ideas might catch on. All we’ve really accomplished is removing a group of (presumably) reasonable, thoughtful people from the discussion – and I don’t see how that would be helpful.

An alternative to ignoring a bad idea is to confront it in a dispassionate manner; i.e. present good arguments as to why the idea is bad and address the arguments made by supporters of the bad idea while avoiding appeals to emotion, ad hominem arguments, other logical fallacies, etc. This allows us to address the bad idea without (hopefully) ramping up the anger and polarizing affects discussed in the video.

So, should we confront all bad ideas? I don’t think so; plenty of bad ideas are marginal enough that ignoring them is probably the right answer. For example, it is possible today to find on the internet people arguing in favor of a return to National Socialism. In much of the world, this idea finds little traction (unlike in the 1930s). Therefore, it may be the case that today, ignoring this attitude is the best approach. However, if this attitude ever begins becoming mainstream, then we should switch strategies and address rather than ignore the idea.