Post-argumentalism is your resolute conviction that the time for argument is past. Firstly, it is the feeling you get when you realize that "you can't argue with crazy", or with "stupid", or with "crooked". Secondly, it is the policy you formulate in the wake of that realization.
This does not mean that you cease to use argument or debate, as a tool, on selected occasions. Rather, it means you have given up the idea that you cannot push ahead with your plans unless you first persuade your enemy of something. Really, if there is something you need to be doing, why not simply go ahead and do it? What are you waiting for -- permission from your enemy based on the outcome of a discussion? This is your enemy, for heaven's sake! How much consideration do you owe them? And just who the hell do they think they are anyway, the official gatekeepers of the turnpike?
The point of post-argumentalism is to reach your goal by some other method than intellectual persuasion of your enemy. So if you know you are right, just act like it, and forge ahead with your plans!