Payment of outstanding amount

This happened a year or two back. During February they phoned me and told me that my contract with them expired and if I wanted to renew. I told them they must not renew my contract. They did not however cancel my stop order and kept on subtracting from my account. I then sent them an e-mail with the request to stop the contract. They phoned me during May and told me that they will cancel my contract but needed a two month period to determine if their was any outstanding costs. A gave them until August and them canceled their stop order at the bank. During December I noticed that they subtracted R 300, 00 from my account. I requested the bank to stop the payment of the money as I gave them enough time to recover outstanding costs. Their should also be no outstanding amounts as this was paid with a stop order by the bank (FNB). During April 2015 they handed me over to dept collectors for the R 300, 00 who told me that if I do not pay this amount they will have me blacklisted. I feel that I do not owe Virgin Mobile anything and will not pay them.