But instead they have fallen back on one simple rhetorical device, which is to accuse anyone (even those who supported Leave) of being "anti democratic" or "opposing the will of the British* people" if they dare ask any pertinent questions or heaven forbid express the view that they still think Brexit isn't a good idea or might result in some difficulties.
In a democracy, democratic decisions can and indeed must be subject to debate and disagreement, and must be able to be challenged through democratic means. Anyone saying otherwise is essentially promoting a dictatorship of the (in this case slim and some polls indicate diminishing) majority. Also, this is how the Brexiters got a second referendum on EU membership in the first place (they argued for a referendum, persuaded the Tories to make it their policy, and then enough of them voted for a Tory government).
After a general election no-one says that opposition parties have to stop opposing policies of the governing party because that's "the will of the majority of voters". Why should a referendum be different?
Also, any vote (general election or referendum) is a snapshot of people's views at that moment in time and views can change. If there'd been a referendum on the invasion of Iraq, a slim majority (if opinion polls of that time are accurate) would have voted in favour. At that time those who opposed the invasion e.g. the then LibDem leader Charles Kennedy, were attacked and vilified in particular by the right wing media and political opponents (sounds familiar?). Within a few years, public opinion changed dramatically and Charles Kennedy was sadly proven right (I say sadly because of the many horrific consequences of that war).
The anti-democratic Brexiters appear to have such little faith in their own arguments that the only way they think they can prevent any change in public opinion is to stamp down on any legitimate questions or debate. I wonder what are they scared of?
The only way that Brexit can be stopped is through democratic means e.g. a general election, a referendum on the terms of any final deal etc, so if you oppose that, you oppose democracy when it doesn't align with your views. If the Brexiters truly believed in the brilliance of leaving the EU, then they wouldn't fear any democratic challenge at all, because they'd be confident it would fail and their arguments would triumph.
* of course a majority of the British people didn't vote for either side in the EU referendum as a sizeable minority chose not to vote (so we can't be sure what they think), some Brits were prevented from voting (including some of the estimated 2 million living in other EU countries), and up to 1 million foreign nationals (Irish and commonwealth citizens resident in the UK) were able to vote in the referendum (caveat: I don't know how many foreign nationals voted).