People who say “It’s not up for debate,” or “The debate is over,” are simply telling you that their position is weak and doesn’t stand up to scrutiny.
“It’s not up for debate” = “We can’t win if you make us debate this.”
There’s no such thing as a debate being over. Debates are critical exercises.
Two important outcomes come from nearly all debates. Either someone makes a strong case and reaffirms their current position, or someone learns they are wrong, or missing information, and rethinks what they think they know.
Both are valuable conclusions.
There’s always something to be learned from debating, which is why it’s such a valuable exercise.
My suspicion is that those who say “The debate is over” would stand to gain the most by engaging in the debate.