ive been overchargerd by unisa after cacelling

I registered for 3subjects and 2 practical subjects and the material was delivered late. the 1st assignment was due within a week and relised that I wont be able to make it. I decided to cancel my studies online. I kept on checking my cancelations but the subjects where still registered on my name but when I cancel it will confirm that the subjects where cancelled. a week later I checked again and found that still the subjects where not cancelled and I called them and was told the will cancel but nothing happened and final I managed to cancel them online on my own. I payed in the deposit of R3000 for all the subjects and after that I was owing unisa R1500 extra for the subjects which I cancelled and I kept on phoning them and couldn't go through till I was handed to debt collector whose called me with the amount. I payed the outstanding amount and feel I was ****** some how and need further investigation and be refunded my money.