MTN contract cancellation

I’ve cancelled my contract with MTN last year but they still continue to bill my account and the billing is not even the same monthly….I tried calling their call centre and going to their shops but am not getting any assistance from either of them…Everytime I call they will pretend as if they are assisting me but they will just put the phone on hold until you give,the sad thing is am longer using that sim card they are busy billing me for, meaning MTN has been taking my money for the past nine months for nothing.