Billing after contract terminated.

I migrated my number to MTN and cancelled my account with Vodacom on the 15th of April. Vodacom then charged me for the month of May. When I contacted them, they stated that an error was made but assured me that I wouldn't be charged again. This morning I was charged for the month of June! I've called cancellations and accounts a no one can help me. I spent almost an hour on the phone this morning being pushed around from department to department without a solution to my problem. How can Vodacom get away with charging for a service that they no longer provide? And why can't anyone seem to fix this problem. Shocking service and this is exactly why I left Vodacom.