Paid up contracts and still get billed for them

I have made payment at Vodacom Heidelberg to pay up contracts a few months before the contracts ended. I am still billed with these contracts and vodacom keeps telling me my account is in arrears, but the arrears amount is due to the fact that vodacom bills me for contracts I specifically asked to be paid up in December. Your administration sucks, after numerous phone calls and emails my account is still not rectified. I will not recommend vodacom to anyone if this is the way they handle business.