Mtn not wanting to cancel my contract since October 10

I had contract with mtn, on September i finish my contract with them, on 10 of october i call mtn to cancel contract since am done paying them, guess what since October calling mtn and now, my contact is not yet cancel, on top of that someone at mtn upgraded my contact on behalf of me, without my concern, now mtn is putting data which is R210but when i call mtn the saying am owing them R671, when i ask, is for what that money the are saying for data, 20gig for R671Mtn is making me full, and the doing *****, the are saying the having system problem,since October on top of that the are billing me for their mistakes please help