Refund after being overcharged NOT done as promised 3 times

@Vodacom, I am at my wits end with you guys.Since the 6th of September I've been on calls with your Client Services Consultants - a 40 min call at a time, so sort out the amount you have been overcharging me incorrectly since Feb! I have reference numbers, everything, but the credit just does not happen as the different consultants confirm and promise it will happen. Monday's consultant promised me an SMS to confirm that the credit will be processes within 24 hours....NOTHING. What must I do next to get this done. To add fuel to the fire, your Sales Consultants (and the outsourced ones) are phoning me around the clock to try and get me to upgrade! I have been a solid client that never miss a payment for years. What can I do to get this issue resolved??? I refuse to go through another call on my time and account about this again. Please!!