Me: "Hey, I agree with you in principle, but your arguments are bad. You're starting with the wrong premise, and then building fallacies upon fallacies. Check your facts and your principles, and make sure they're all sound before you try arguing against that guy/idea/whatever."

Them: "Shut up you commie/fascist/Trump supporter/Trump hater/feminist/men's rights activist/hippie/corporate stooge!"

Me: "OK, then. Let me know when you're ready for rational discourse."