Here is what I believe:
Yes. Some ideas are dangerous. I for one think many of Nietzsche's ideas are dangerous. But some are interesting and important.
But trying to suppress the dangerous idea actually does the opposite. I see this as the social version of the "Don't think about <x>" phenomenon (where we are in-fact compelled to think about <x> while trying not to thikn about it).
The best thing to do with dangerous ideas is to simply treat them like every other and reject it like we do most ideas any way (since most ideas are not that great in the first place). Calling special attention to it makes things worse, because it treats it as something special, and gives it more attention and focus.
Once it blows up to that level, there is nothing we can do but try and contain the damage done by those notions, and show why they are unsound.
An other issue, is that many times people will twist an otherwise innocuous idea into something dangerous and rail against it. This again gives rise to a dangerous idea and gives it more life.
A lot of other times, an ideas is dangerous to some for the very reason that they are good for people in general. The right for women to vote is one of those ideas. It threatened a lot of men, but has been better for us overall. The fact was that the prodjudice against women voters was holding society back, but it still yeilded many benifits to those in power. It was a dangerous idea to them.
That is why I believe that ideas should simply be let to run their course on their own merits. If we label ideas as "dangerous" we start distorting the dynamics and give it more power. Perhaps that is just part of the game. But I don't think it has the effect that people who do the labeling want it to have.