I am quite tempted to ask this question because I have committed this a few times recently (e.g. Is this a useful concept in trying to solve the Riemann Hypothesis? Or am I still to naïve? , Why isn't this a bijection from N -> R? or Formulating a philosophical observation in math ...)
On one hand, those are people who have gross misconceptions about basic notions in maths, which means that the questions they ask are in most part nonsense; on the other hand, they are not necessarily deluded people and it is hard not to notice their genuine interest in the subject.
So, is it better to engage with them, even if the discussion may stray a long way and end up mostly useless for anyone else (possibly also useless for the OP and for the person providing the answer - sometimes it feels like talking to a wall), but with a potential benefit that they may learn something new, and they may feel that their interest has been appreciated?
Or, close down such questions quickly without engaging, which doesn't waste anyone's time but may breed resentment and come across as rude - as it is hard to close the question with a reason meaningful to the OP without engaging the OP at least to some extent?