MTN charging for a SIM card IM NOT USINg

I don’t understand how I cancelled my contract sim with mtn because I didn’t get the data I signed up for! I stopped using that SIM card and MTN is still billing me and now I owe 800?? How?? I will not be paying for something I’m not using! Cancel that contract that’s what we agreed on the last time o talked to your consultants! U take people for a ride!!!!!!!!