Is there any term in English for a person who starts a debate, but once he is presented some valid arguments and facts that he cannot rebut, he quickly retreats saying that the further debate is meaningless because his opponents are just stupid, stubborn, dull, etc. (An idiom describing such an action is also welcome)
Weasel out — "to evade or avoid a job or responsibility".
Jim always tries to weasel his way out of the debate.
It's not exactly the same, but I think it is the closest you are going to get.
I'm not aware of an idiom that wraps up all of the question's requirements, which are
- a person who starts a debate, then
- retreats, quickly, after
- some valid arguments are given that
- he cannot rebut, then
- says further debate is meaningless because
- his opponents are stupid, stubborn, dull.
Several terms are available that cover one or more of the requirements; but perhaps pettifogger, "someone who quibbles over trivia, and raises petty, annoying objections" or "an unscrupulous or unethical lawyer, especially one of lesser skill" may come closest to the sense of some of the requirements. Also consider slanderer, "one who slanders or defames the name or reputation of another person", and the phrase ad hominem argument, "a type of fallacious argument in which the attempt is made to refute a theory or belief by discrediting the persons who advocate that theory or belief".