I’m starting to think that the correct way to address these types of things is with a simple, possibly even loving, “that’s the dumbest thing I’ve heard”.
A lot of poorly reasoned opinions and ideas come from socially validated sources, they parrot the opinions of people they identify with. Showing that you won’t agree to get along, might work as a counter both to their spewing the shit as well as maybe letting them question why they hold that belief.
Might also make them stop coming to you with these types of opinions, which may or may not be what you want.