Almost 4 years and no action from Vodacom

It's been almost 4 years now that I've tried sorting out my Vodacom account. I had an unauthorised contract on my name. Vodacom cancelled the contract but never credited my account. After being handed over to numerous debt collectors who took the time to look at my account and send it back to Vodacom and still no action from Vodacom. What is my next step? Sue Vodacom for defermation of character?