Should you respect the feelings of someone you believe is being deliberately ignorant?

Let’s say there is common knowledge out there that any one can access. Say, for example, information that the world is a sphere, not flat. Yet someone ignores all that information and insists that the world is flat.

You think the person is an idiot. Worse, a willful idiot. When you talk to this person, should you respect this person? In what way should you demonstrate that respect? Should you avoid telling the person they are ignorant? Should you avoid saying anything that might hurt their feelings?

If so, why? Why should you care about the feelings of someone who prefers to remain ignorant?

Let’s say this person is running around proselytizing to others that the world is flat. Let’s say other people are listening and agreeing. Does this change whether you respect this person’s feelings? Or can you still respect the person while attacking their ideas as “stupid” and “ignorant” as long as you do not attack the person in this way?

Our community manager has asked that we try to remain cognizant of the feelings of the person whose ideas we are attacking, and to respect that person. I’m not sure that is doable if you believe the person is being willfully ignorant. Is it possible to attack a stupid idea without the person who has the idea feeling like you think they are stupid?