Suppose hypothetically you've decided there's simply no point in trying to explain things to a certain user. But he's asking questions about something. Community-norms-wise, which alternative seems more appropriate?
(i) Simply not replying to his questions
(ii) Explaining why you've decided there's no point in trying to explain things to him.
On the one hand just ignoring the guy seems rude; on the other hand calling the guy a moron also seems rude, hence the question. (Don't bother suggesting that I simply try to answer his questions - that's not gonna happen.)
(Talking about the OP here. In another thread he recently spent a long time insisting that operators on a Hilbert space satisfied $||ST||=||TS||$, after I'd given him a very simple counterexample. He said it was a proposition in a certain paper. Here's what really pissed me off: He repeatedly insisted that he was not leaving out any hypotheses, the proposition applied to any two operators $S$ and $T$. A few people with access to the paper joined the discussion and explained that no, there were in fact fairly intricate hypotheses he was leaving out. Saying he'd been lying about the statement of the proposition would be awfully strong, but in fact what he was saying was simply not so, and clearly not so. Pissed me off - also seems somewhat bizarre, claiming a paper says something that it simply doesn't say. To be charitable, I suppose it's possible that he really did think that the proposition applied to any two $S$ and $T$, but if so it seems like there's not much point trying to explain things to him... Then when he finally conceded that it wasn't so it really didn't seem like the counterexample had anything to do with it, seemed more like he was just going along with the consensus...)