NASHUA ACCOUNT CANCELLED AND THEY STILL INVOICE AND BILL CLIENT

During October 2014 I cancelled my Nashua phone contract and sent the cancellation to the relevant persons and departments, I have since then been harassed and phone calls are being made with regards this matter and every time I have to send them the cancellation email that was sent in October 2014 already. Nashua has been bought out by MTN and still they are reluctant to acknowledge my correspondence. I get sms's of ignoring they attempts to resolve but when they ignore my correspondence and do not acknowledge the cancellation what else do I need to do to get tis resolved