One of the main roles of an executive assistant is that of gatekeeper. Controlling access to the boss is an extremely powerful role, and your potential employer wants to know that you can handle it. A good answer would be something like this, “First, I’d try to understand the situation so I can know why that person is trying to reach you and why you’re trying to avoid it. Then I’d see if there’s any way I could help that person myself. If not, I’d just continue to politely take messages without making any promises on when you’d be available to respond.”