My account still being debited after my contract

on the 04 July 2015 i cancelled my contract and i thought everything OK, then come the day of my salary i was surprise to find out that my account was debited, i called mtn they said its been cancelled then i reversed the debit order something happened in august and September i had to reverse those debit orders now I'm on pay as you go every time when buy airtime then they give me this wow bonus which expires mind night that day to my surprise when they take their remaining wow bonus they take my air time as well and the follow day i have balance airtime regardless that i had loaded my own air time. when i ask the client services they tell me i was on Internet and it took every thing so my question is why does it happen when i have that so called wow bonus? it happened several times and every week i have to call mtn. everything is a mess after that strike. I'm going to pot my number to another network this is enough I'm loosing instead of benefiting. i have been with Mtn for 16 years and their service has gone from bad to worse. I'm very disappointed