Some express the view that anyone can express any opinion and legitimize it with "I have the right to my opinion!".
Well, you may have the right to believe "Mercury is natural and therefore can not be harmful. Ergo, replace air with mercury"... but that does not mean anyone must accept it as legitimate.
You can have it and express it, but it's still plain wrong.
Other ideas, like "kill newborns because of what some believe about their grand-grand-grand-[…]-grand-grand parents"...
it's an opinion that is theoretically possible to hold, but even expressing it as if valid generates acceptance and thus should not be accepted by anyone, so the immediate response should "then my opinion is that I now refuse to have anything to do with you, until you stop promoting that".
Important that it's explicit, so they know it happens because of what they choose to do, as with all boycotts.