Remember that you're talking not just to them, but to everybody who reads the exchange in future, and some of those people will need those calm words to turn away from the rabbit hole.

This is the most important part. If they're arguing in bad faith, or they're just stubborn and stupid, you're not going to change their mind even with the kindest and most comprehensive response. But there are lots of people watching, and a lot of those people are trying to decide how they feel about the subject in question.

The best thing you can do is stand out as the voice of reason in an exchange. Your conversational partner might not be convinced, but onlookers will be.