Billing issue and no proper investigations

I am very disappointed with the Mtn service.. I have a two contract with Mtn..the one came to an end, we ask to upgrade to cheaper package but this was not done.. We have been buying airtime to purchase Google play and now they are also charging the account ..this has never happened before the upgrade.. we have been always buying additional airtime to buy Google play.. seems their system is defective and there consultants is not aware of what's happening , they just tell you to unsubscribe without investigating properly