Secondly it is arrogant in the extreme to believe that you are critical enough to see deniers for what they are but others must be protected from having to form a reasoned opinion.
I don't have specific evidence but banning things, in general, is counter-productive. It drives the activity underground. Examples include prohibition and narcotics in general.
In the case of Holocaust denial it also suggests a lack of confidence by the establishment, that they fear that the deniers have a plausible case. I, like most people have seen mountains of evidence showing that the Holocaust did happen. All bar a few cranks and simpletons having taken the trouble to look at both sides would surely come to the conclusion that the Holocaust did happen and see the promoters of denial for what they are.
Denying people the opportunity to hear the deniers' case promotes the suggestion that there is a conspiracy concealing the truth from the public.
"[W]e recently banned anti-Semitic stereotypes about the collective power of Jews that often depicts them running the world or its major institutions." – Mark Zuckerberg (CEO) & Sheryl Sandberg (COO) of Facebook
Look, I point out the above primarily in jest. (There is irony there, no?) But the other point that I'm making is the above will not be missed by conspiracy theorists.
Would you mind linking to the studies you have in mind? In thinking about the likes of The_Donald and its current, banished form.