I think he should be allowed the same freedom of speech as the rest of us to advertise and evangelise their own beliefs, opinions and values.
If denied this privilege, future deniers can easier and with some justification claim that "history is written by the victors" (interesting google), and possibly even re-inforce their cause.
Better, I think, to let them "publish and be damned" so that future historians can themselves see the ridiculous arguaments being put forward as an alternative history.