Cancelled account and I still receive statements

I paid up and cancelled my Cell C contract on 11/06 and at the end of July I still received a statement. I contacted them and they said that I must pay the amount and they will credit me as I will only have to pay for the 11 days I was still with Cell C. I requested my credit on numerous occasions after my debit order went off but without any luck I didn't receive it. I thought I will leave it until the end of August to give them time to credit me and come end of August I received another statement that will be due for payment but I don't even have a contract with them anymore. My number was ported back to Vodacom due to the bad network Cell C gave me and even after numerous complaints they still did absolute nothing. I contacted them again after receiving this statement and they now say I only cancelled my contract on 21/07. This is unacceptable and I firstly want my credit and secondly I will NOT pay them at the end of this month. Their service is extremely bad and they do not give a **** to anyone, all they care about is taking your money. I am still waiting for them to get back to me, but no one gets back to me with my credit. Please assist!!