Pathetic Service on Contract Cancellation

Pathetic service !!! I left the country last September on the 11th and on the night i flew i phoned Vodacom to cancell my contract which only had 1 month left. I said i was happy to be debited for that last month, which they did but THEN THEY KEPT DEBITING my account which i then had to cancel with the bank. I am still receiving threatening emails and notice to hand me over after numerous emails - even to the CEO. They actually owe Me money back for the money they took off after i had cancelled!! How do i get this to stop without being black listed. I have tried everything. Absolutely pathetic. Especially after i have never missed a payment to them after about 20 years of having cell phone contracts with Vodacom only.