Vodacom unable to solve/settle overbilling on my account

After 5 months Vodacom still have not resolve the overbilling on my account. I changed from contract to pay-as-you-go on one number and downgrade another number, but vodacom still billed me for the already pay-as-you-go number. After numerous calls week after week, and promises by Vodacom to credit my account, my services was still cut due to unpaid billing according to Vodacom. After Vodacom agreed to credit me with the outstanding error , Vodacom only partial credit the amount, leaving the rest just to be disputed once again. Another 5 months of discussions?? Please help!