The unreasonable insistence that a concept cannot be defined, and therefore cannot be discussed.
"Loki is a trickster god in Norse mythology, who, legend has it, once made a bet with some dwarves.
It was agreed that the prize, should Loki lose the wager, would be his
head. Loki lost the bet, and in due time the dwarfs came to collect the
head which had become rightfully theirs. Loki had no problem with
giving up his head, but he insisted they had absolutely no right to
take any part of his neck. Everyone concerned discussed the matter;
and, one could suppose, they are discussing the matter still. Certain
parts were obviously head, and certain parts were obviously neck, but
neither side could agree exactly where the one ended and the other began. As a result, Loki keeps his head indefinitely."
Dealt with by challenging for a reasonable working definition and showing that the other party is being unreasonable and avoiding the argument.