People who promote them have as much a right to speak as anyone else. Also it is important to consider that it is better to refute their opinions than to silence them - which encourages martyrs and conspiracy theories more than anything else. If there is something that should be done, it would be to reduce censorship as much as possible. The way I see it, there are basically four options on how to deal with unwanted material. Now, warnings are not part of censorship per se but are used on the Internet as a way to escape censorship.
Most controversial websites use the second, sometimes the third option.
This article is an example of the third option. I have warned you, the reader, that my opinions may offend you, inside the article itself. Perhaps this particular example seems unnecessary, but be reminded that my text may have been offending at the beginning of the century, and possibly censored. Putting a warning before the material is usually a waste of space as well as time for the reader. This would be perhaps a good compromise, but I would not advocate to make it mandatory.