If a low reputation user posts a question about the Holocaust, rather than being immediately posted, it should receive a notification telling the user that the question will remain visible (and editable) by that user, but will not be published until it’s approved by either the community or a diamond moderator.
The question should remain invisible even if the user edits it. That way, users won’t be able to circumvent the filter once it’s been triggered. In addition, users won’t receive any warning before the question is submitted. This will avoid people trying to misspell the word to circumvent the filter.
Update: I had a look at a sample of questions, based on which ones I flagged. It looks like a sizeable proportion lasted longer than 2 hours (if you define it as precisely 120 minutes). While the questions I flag may be a biased sample (for example, I can only flag questions which haven't already been deleted, and I'm more likely to flag questions which were posted when Australians are awake), I think that "virtually all ... within 2 hours" is an exaggeration.
12 questions that lasted less than 120 minutes: 6 minutes 16 minutes 28 minutes 54 minutes 55 minutes 1 hour, 6 minutes 1 hour, 6 minutes 1 hour, 10 minutes 1 hour, 10 minutes 1 hour, 13 minutes 1 hour, 33 minutes 1 hour, 40 minutes
14 questions that lasted more than 120 minutes: 2 hours, 10 minutes 2 hours, 42 minutes 2 hours, 43 minutes 2 hours, 52 minutes 2 hours, 57 minutes 3 hours, 31 minutes 3 hours, 32 minutes 3 hours, 50 minutes 5 hours, 32 minutes 5 hours, 57 minutes 6 hours, 8 minutes 6 hours, 28 minutes 6 hours, 33 minutes 8 hours, 20 minutes
While this is a limited sample, it already results in a total of 75 hours of Holocaust denial on skeptics.SE, with the average question lasting 172 minutes and a considerable number lasting for multiple hours.
Update 2 March 2019: In an answer to this question, fredsbend asks whether holocaust denial posts are a significant proportion of trolling posts. To examine this, I examined my flagging history, examining "helpful" rude/abusive flags, and looking only at questions. (This may not list some questions listed above, as I may have used custom flags on those questions). There are 18 such questions. Questions that explicitly mention the word "holocaust" make up half of such flags, and make up a narrow majority of questions that ended up being deleted. I have made a total of 294 flags overall, but that's including flags for comments, flags for answers, declined or retracted flags, flags about "" banners, and various other matters.
Mentions the word "holocaust", either in the title, the body, or with the tag holocaust (9 questions): https://skeptics.stackexchange.com/questions/43439 https://skeptics.stackexchange.com/questions/42754 https://skeptics.stackexchange.com/questions/39608 https://skeptics.stackexchange.com/questions/38322 https://skeptics.stackexchange.com/questions/38317 https://skeptics.stackexchange.com/questions/38264 https://skeptics.stackexchange.com/questions/36478 https://skeptics.stackexchange.com/questions/36336 https://skeptics.stackexchange.com/questions/36103
About Nazi Germany, doesn't mention the word "holocaust", one question involves an individual primarily known for his role in the holocaust (2 questions): https://skeptics.stackexchange.com/questions/42900 https://skeptics.stackexchange.com/questions/2260
About Jews, but not about Nazi Germany (2 questions):
Are American media controlled mostly by Jews? (2015 edition) (marked as duplicate of an open question)
Not about Nazi Germany or Jews (5 questions):
Did the former prime minister of UK say this about the Koran? (still open)
Did Adria Richards tweet "Black people CANNOT be racist..."? (still open)