Bad service

I had a month to month contract with Telkom of which when I started with it they took money from my account on a "pro rata" basis. When I was not receiving satisfactory service from them, I cancelled the contract and I was not owing so I didn't have to pay them anything and I didn't owe them anything. Funny thing is when I went in store to get a sim card from them, I was flagged as an owing customer and they couldn't assist me. How is this even possible????? I need intervention asap.