i think they should be free to say what they think and then face any consequences.
as said above i am of a mind to think that most of the deniers believe that something happened, just not to the extent claimed, and that the holocaust is some sort of jewish sympathy media machine. mostly that the camps were built and used for forced labour and re-education rather than mass murder.
my thoughts are otherwise having seen a couple of the camps (also khatyn memorial as well), and as said here there is plenty of historical evidence of operation reinhard including written, videographic, photographic and personal.