Still being billed after contract cancelled!

The day that my 2 year contract with Mtn matured, I called in and cancelled all services with them. Following on this the 1 number was ported to Vodacom successfully and the other was discontinued. Now over 3 months later I'm still receiving invoices for both the numbers. How stupid are you to see that numbers are no longer on your network, but still continue billing for them? I demand that you rectify this immediately before I take legal action against you, as this is negatively affecting my credit rating! I have also put a stop order on there requests and not even a call to question why they haven't been paid in 3 months, slow system or slow staff? DOESN'T MATTER FIX IT!!!