I'm a moderator on one of the non-toxic parts Reddit (r/AskHistorians). We've dealt with holocaust deniers for a while and find that the best policy is to shut that shit down.
Here's a short thread on what I've seen/dealt with.
They hold their more crazy theories at bay and frame it as good faith questions.
Often all it takes is browsing their user history (or lack thereof as many are sockpuppets for people we've previously banned) to find their real agenda.
The method that works with holocaust denialism is shutting that shit down. These folks take advantage of anything short of that
reddit.com/r/AskHistorian…
The only way AskHistorians has avoided becoming a total cesspool is our policy of *heavy* moderation.