1keep in mind – as nasty as this person appears to be right now
Also keep in mind – as nasty as this person appears to be right now, this is still a living, breathing human being with friends, family, fears, challenges and shortcomings. Don’t escalate the dialogue with confrontation. Instead, allow them to have their opinion. Respect their opinion, or at least give the appearance of respecting it (no matter how off-the-wall crazy it might be.)
Answer calmly, rationally and above all NICELY and politely. Often they will respond to your kindness with kindness of their own, and may even offer an apology. If they don’t respond, don’t worry about it. Your goal is not to win them over; it’s to appear as the same rational, intelligent, level-headed expert your readers expect you to be.
That’s why you’ll always keep in mind that your audience is reading this interaction. If you sink to the level of your detractor, you will lose respect and you will lose readers. But if you fail to take the bait and remain the same cool, calm professional you are, your readers will respect you even more.