A fallacy (and a type of AdHominem) where the speaker declares that his opponent is unworthy of the debate. Sometimes, this assertion is a justification for the speaker withdrawing from the debate (often DeclaringVictory?), other times it is a form of intimidation or showmanship. And sometimes, like all fallacies, it is valid.
It's not a fallacy
Calling a person or proposition unworthy of debate is not a fallacy at all. It's not the kind of thing that can be a fallacy, because it's not reasoning, and therefore of course not invalid reasoning. People and propositions can indeed sometimes be unworthy of debate. The ignorant do often talk nonsense. They do often set up foolish criteria by which to reject the vastly more informed and sophisticated thoughts of experts. The rules of the game set up by the ignorant often preclude even the possibility of acquiring knowledge.
Calling someone a genuinely stupid person stupid to his face is just incompetent rhetoric. True as the claim may be, it doesn't enlighten or persuade.
You may be slightly off point here. The fallacy kicks in when you assume someone's next statement will be nonsense before they begin. Of course, while this is a fallacy in theory (who knows? They might have said something brilliant, and anyway StupidQuestions? are sometimes the most profound) it's often useful to avoid wasting your time (see the doctor example above).
Don't look at me; I just ignore cranks.
Why bother to tell them that you are ignoring them? If you don't get along with somebody, simply ignore them without leaving traces. See also: ArgumentumAdInfinitum Stating that one ignores cranks on a page where that is a relevant topic of discussion, doesn't contradict the act of ignoring them. Now if someone were to do the Wiki equivalent of sticking out one's tongue, plugging one's ears, and saying "nah nah, I can't HEAR you", that would be inconsistent with a policy of ignoring cranks....
See also DogmaticFallacy (because of the examples given above) - last edit Feb05 by gw01.tipp24.de