What is the opposite of "preaching to the choir"?

I have found "Whistling into the wind" online but I do not think it fits because it seems to mean that your words are not heard, whereas the opposite should mean that you're being informed by someone of a 'fact' with which you strongly disagree.

I generally respond, "you're preaching to the choir", when somebody 'vents' to me about something that I too take some issue with, which may be an evolution of the idiom used when you tell someone something they already know, rather than something they agree with.

I think casting pearls before swine is a good opposite. It means giving advice, guidance or direction to those incapable of appreciating or acting upon it.

I say "You're banging your head against a brick wall" or "You're wasting your breath".

You might say “You’re talking to a fence post” (ie, talking to someone who might as well not have ears), if you want to add a slight bit of humor via self-deprecation. More bluntly, you can say “Talk to the hand”. Wikipedia mentions “Talk to the hand, the ears ain’t listening” and “Talk to the hand because the face don’t understand” as variants, but it also call the phrase “quickly dated slang”.