Zone Fitness Unhelpful

Hey I signed up for zone fitness in 2015, I have only made use of the services a few times, however I continued to pay the contract. I did monthly payments because the cancellation cost would be too much. When the cancellation cost became a reasonable amount I was able to pay I made the payment to cancel the contract. It is going for 6 years later and I am getting calls from zone fitness collections agency every day this has a negative impact on me. Everytime I have to explain the same story and I had to send the documentation a few times to show I paid up the account. The agents just tell me they will add a note on the account and have it escalated, just to have another person call me the next day. Sometimes they even call 3 or more times a day. I would really love this to stop it has been a nightmare for far to long. No one seem to be able to assist cause when I call zone fitness they refer me to the collection agency and when I call them then they refer me back to zone fitness. What should I do next?