2 Years after C Cell was paid, they want money

Before my Cell C contract ended more than 2 years ago, I went to the local branch to say I don't want to continue with it after the contract ended. They said that they can't do it, that the contract must first be done. After the contract ended, I went there again and was told that they can't do it, a certain form had to be filled in, and this form had to be faxed to Cell C. Their printer was not working and they said that they will email the form to me. This form was never emailed, and I went to the branch two more times. In the end they charged me for 2 more months after the contract ended before I could get it stopped. I didn't want to pay the R150 for the extra 2 months, but they threatened me with blacklisting and I paid the R150. Yesterday I got a email from a debt collector of Cell C, demanding payment for R150. Cell C accounting department must really be in a sad state, that they have no clue on who paid them or not. More than 2 and a half years after I paid them, they want me to pay them again!