Well, Irving was debated at the trial (and in the book that gave rise to Irving's claim of defamation), some biologists set up archives of creationism-debunking material and also appear fighting rearguard battles at the hearings on school curriculum, and creationism certainly shouldn't be made 'legal' by calling it and allowing teaching it as science. These analogies aren't too helpful.

you don't fight an ideology by making it illegal

You can fight its spread that way. Holocaust denial is not just crackpot history and ideology.

_{*Lunatic*, n.
One whose delusions are out of fashion.}