Use the phrase "End of"
One of the current uses for this conversation stopper is in the endless debate over Brexit and its most frequent form goes along these lines:
"The people voted for Brexit. Get on with it. End of"
"End of" does not just signify the completion of the comment. It is an intensely arrogant statement. The words are meant to say there is nothing more to be discussed, my argument is utterly infallible and nothing you say is of the slightest relevance. There is no need for me to add anything because, as Rik Mayall used to say - "A - I said it and everything I say is interesting per se; and B - well there's no need for B because the A was so great"
In the case of Brexit any questions over, say, the border in Ireland or the political rights of Britons living in the EU are swept aside as unimportant. If you query the many statements made during the referendum about how easy it would be get a trade deal there is a sullen silence. The magic phrase "End of" can be invoked at any time to shut down dissent much as the medieval church could end any awkward questions with the accusation of heresy.
Of course it is not just key political questions that are stifled at birth in this way.
"I think that winger was offside"
"Yeah? No, he wasn't. End of"
You can't talk properly to people who respond in this way. Sadly, you have already had to listen to their viewpoint before they hit you with their knock-out blow; if you had known they were going to be such total dorks then you could have walked away at the start.
Perhaps one could reopen a shutdown argument thusly
"Climate change is caused by too many polar bears melting the ice. End of"
"No, you are utterly wrong and moronic to boot. Beginning of"
but I doubt if this will catch on. The only solution is to ignore them. At least they have stymied themselves because, once they declare a subject closed, then they cannot reopen it. That's about it for this topic. End of.