Mtn contact canceled but still been charged

I will like to talk about the bad service I had with mtn I had a contact with them I then wanted to cancel the contact so I call customer service and there gave me a amount to pay I paid the full amount my Sim was blocked and there gave me a date when it will b canceled I then saw that there have been taking money from my account now I own so much for nothing because I canceled my contact and the Sim is blocked the service is bad I will never take a contact with them