I typically reiterate that I’m so sorry, but she is unavailable until x date, and repeat my request for advance notice as gently as I can, while avoiding the phrase “short notice.” What else can I say when the harsh truth is, “She’s an incredibly busy executive who is interviewing candidates/leading committees/teaching/in meetings/on calls/in planning sessions all day every day for the next 2-3 days, and you should have asked for an appointment when you knew you were coming to town a month ago! I’d love it if there were a diplomatic way to be more stern. When you see people constantly doing this, it’s natural to develop a desire for a way to set them all straight in some broader way.
Most of the time, we need X amount of notice to get new appointments on her calendar.” Say it in a sympathetic tone, but spell out the situation.People never trust an accommodating man with important things.That may sound harsh and cynical, but check it up in your own experience.But I wouldn’t worry about finding ways to be more stern.
The substance of your answer is already stern enough to make the point (we can’t do the appointment because this isn’t enough notice), and I think it’s probably better for your office’s relations with people if your tone is warm, even while you hold firm on the ultimate answer.
If you have a severe illness, for example, you turn to the busiest, most exacting doctor in town.