I understand the degree of frustration involved Crash. My ire gets up still when I read such nonsense even when I'm not involved and t h i n k I'm ignorning the nitwits.
However, the rules say that you can't call someone a liar. So you either stop wasting time with wackos or you carefully keep simplifying things:
He says "There you are, clearly asserting the sky is yellow..."
You have to (it is harder in the real case) get it down to fewer words and posts. Ask -- "In that post of mine, where is the word "yellow".
At some point, if you want to waste time with such a game, you may have to resort to going over the rules of grammer. It may be that the individual is actually not lying !!!! They are so narrow minded and focussed that they misread even a fairly simple bit of English.
You can see if you can get them to look at each word of your posts and show how they read them. Personally I think that this is a waste of time with a number of the utterly crazy (or at least working hard to appear to be-- perhaps just in order to play with you) here.
Given that they aren't just playing the fool then you can make their own posts prove the lie by getting them to say something as clear as in your simple example.
You will have to have unbelievable discipline to control the wild gyrations onto other issues that they will resort to but it would be fun to watch.