I always go with: It is your job to answer. The extended version: If you don't like it you can search for a different job where you won't get questions from me.... but there probably will be others with similar questions or even harder ones. etc basically go on endlessly about the hypothetical them working someplace else. Next time it happens I return to the subject.

Then a coworker showed an even more sophisticated version of the same joke. He created a nearly identical relationship but he would look for job offers matching their resume and pitch them. Look, this is closer to where you live and the pay is better!