Owing Mtn after paying them up

I had a contract with mtn for Lenovo lap top +a router with data every month, I paid up my contract, however I they were still deducting money from my account, I went to their outlet @Maponya mall and I got help to cancel the top up for the sim card,now mtn is saying I'm owing them for 9 months what I'm owing them because I did paid everything please fix my name not happy with mtn at all.