While no one can disagree that such behavior is rude, I would not suggest a blanket determination that the agent shall not be allowed to show your property.
First, it does not make good business sense. If that agent has the right buyer client to purchase your house, you want and need that buyer. If there is a real buyer for your property attached to that agent, we want to get that buyer in your property. Second, most real estate laws on agency in most states , as well as association ethical guidelines, require that we be fair and honest in our dealings with customers, even when we are not their represenatives. If the buyer wanted to reschedule, and had an exclusive buyer agency relationship with their agent, the denial of such appointment may be construed as a legal or ethical violation. As an agent, there has been more than one time that I have had to struggle with a difficult agent on the other side of the transaction. It's my job.
If you are a seller, ask your Realtor to follow up with the agent who missed the appointment. This is an example of where neutral party intervention brings value to the table for the client. The lisitng agent can inquire the reason for the missed appointment. If there is a potential buyer attached to that Realtor, the listing agent can pursue a reschedule. If not that buyer, maybe that Realtor has another buyer. It then falls to the listings agent to follow up and reconfirm any appointments made by that buyer agents reinforcing the expectation of a courtesy call in the event of any changes.
Unfortunately, Arlene, it happens. I just had an agent schedule, cancel, reschedule and cancel again. We received the phone calls, but the seller was still highly inconvenienced by this. It is the minority of agents, yet it rings out loudly in our memory as a result of the frustration.
Rely on your lisitng agent to follow up, find out the reason, and reinforce the need for communicaiton from the offending agent.