Zone fitness lawyers harassment

I’ve had a gym membership with Zone fitness Westgate Mall for 2 years the two years was about the finish and I went in and requested that they should terminate the contract after the last debit order, the woman helped me and gave me a cancellation letter. Months later I’m getting calls from a company call Nimbo stating that I’ve got outstanding debit with Zone fitness, I then went into the Westgate Mall branch to query on the issue and they told me I had outstanding amount of R246. I paid the amount of 246 with my debit card and got a receipt and left, months later again I’m getting calls from Nimbo saying that I owe the gym money and I went back to Zone fitness requested a statement from them and it showed no outstanding amount at all but everyday I’m getting calls from lawyers from Nimbo stating I owe the gym money.. I do still have the statement from the gym worth me and a bank statement as proof that I did pay an outstanding amount which I wasn’t even aware of... is there any steps I can take to further because it’s becoming annoying and sick and tired of going to the gym all the time to try and fix this situation