That happens from time to time. Even when you explain to them why many times that is not possible. Every now and than you have to remind your buyer(s) that notice is often required. For example, my listings I am always at the showing. So your buyer(s) need to know you have to coordinate schedules with other people.
But realtors may already be booked and can't do it on short notice.
Besides, the property they want to see may have specific instructions, like 24 hour notice, for example.
Furthermore, we also ask if the callers are already preapproved -- because if not, how do they they know they can afford what they want to see.
And...are they currently working with a realtor? I one had someone call me, and when I asked if they have a realtor, she said she did, but that her realtor was really busy. Oh? And she thought it was okay to bother others?
Yes I have had that, and I always wonder why the protocol for working with a professional is not understood. The only thing you can do is explain how you work and do business, hopefully you can get it across in a professional and pleasant way. If it's another Realtor, it's learning the hard way I guess.