annoying phonecalls from Planet Fitness

I cancelled my contract. they asked for proof. i have emailed it to them at least 6 times. they call me every second day to tell me i owe them money, but it was in fact the other way around. I eventually managed to reverse some of the payments and i now owe them around R20. i have asked them to reconcile the accounts per email but does not get any feedback from. i only get a phonecall every second day telling me i owe them money. and every time i ask them to look at the mails and get back to me but instead i just get another phone call of a new person asking me the same questions. this has been going on for 5 months