Billed for ********** upgrade

Until September 2015 my journey with MTN had been seamless because i want trying to contact them. September I called to terminate my contract as it was expiring, and what i got 3 days later was a ********** upgrade to MTN Sky (the most expensive package). I followed and the matter was finally resolved after endless calls and reference numbers in November. The contract is now terminated, however, MTN still billed me for the ********** upgrade, and they are now sending me smses threatening legal action even though i have called and been told over and over again that the billing will be corrected. They have even disabled my number, and for the life of me i dont know what else to do, I even tried to get their attention on twitter. Please can MTN get its act together.