Vodacom make a fool out of me again

It is 4 months later and vodacom is still charging me more than the agreed amount for my monthly subscription. They have contacted me confirming that everything will be rectified after my previous hellopeter post But it has not. Over and above this they made another mistake as they did not debit the full amount for my December account. After sending me messages saying that my account will be blocked i phoned them to arrange for the them to debit the outstanding amount. One week later and they have debited the outstanding amount twice. Once again leaving me out of pocket. I honestly have had enough of the inconsistency from Vodacom. What can i do to cancel all of my contracts and move to a honest and consistent service provider