"Mismanaged waiting lists"

I was seen in the urology clinic on early this year and put on a list for a cystoscopy investigation which I had the following month. Fantastic. Problem solved. Discharged

However, a month later I was phoned by Patient Access to offer me a date to have the investigation. I told them I had already had it done and told them to remove me from their list. The person I spoke to acknowledged and said they would do so. 

Two weeks later another phone call, again offering me a date to have the investigation. I went through the story again and asked to be removed from the list.

Today, what do I get? In the post I receive a letter of appointment to have the investigation.

How many times do I have to tell the hospital to remove me from their lists? I've had the treatment, I was discharged, I don't want or need to have it again. I wonder how many genuine patients are on the list instead of numbers of people already treated, but unable to get out of the system. 

How do I get anyone to listen to me and take me off this list? I'm fed up with having to keep telling them to remove me, knowing that they will take no notice.