Amazing eh, they are not worrying about politely asking you a dumb question but you are
trying to find a polite way to tell them to stop with nonsense questions.
I act just like you. If someone talks garbage I just can't find it in myself to tell them to stop.
There is no polite way to tell them to stop asking this question. A person who is that ignorant is going to get offended no matter how politely you tell them.
What you can do is change your reaction to their questions. You have the power to not care.
Life is like an ice cream cone, enjoy it before it melts............so just don't care, that's all.
Sorry, I hope this helps.