Pardon me, but why would you even consider it? It would just be a slippery slope to more and more authoritarian anti-speech laws, as the other posters stated, not to mention such a concept would be egg on the face of the Founders, the Consitution, and the basic principles of America.
Not to mention if you institute such laws, people will have their eyes on them, and curious as to why they cant deny it, so they might find information that allows them to find out why people might deny it, which in turns creates more deniers. See what I mean?