check out our:
[Mr. Darcy] was directly invited to join their party, but he declined it, observing that he could imagine but two motives for their choosing to walk up and down the room together, with either of which motives his joining them would interfere. […] [Miss Bingley] persevered therefore in requiring an explanation of his two motives.
"I have not the smallest objection to explaining them," said he, as soon as she allowed him to speak. "You either choose this method of passing the evening because you are in each other's confidence, and have secret affairs to discuss, or because you are conscious that your figures appear to the greatest advantage in walking; if the first, I would be completely in your way, and if the second, I can admire you much better as I sit by the fire."
"Oh! shocking!" cried Miss Bingley. "I never heard anything so abominable. How shall we punish him for such a speech?" (11.11-16)
Darcy can't actually tell Miss Bingley that she doesn't stand a chance, but he lets her know in the only way he can—by shutting down her flirtation through bluntness.
It doesn't work.