MTN STILL DEDUCTING MONEY FROM MY ACCOUNT

We went to Clearwater mall MTN,we changed the contract from my name into my son's name.This was done in August.Now they are still deducting from my FNB account and his acount and despite sending e mail after e mail,they do nothing about it I have blocked it but they are so clever they change the amount..Please who can I contact to sort this out.Phoning them does not help as they never answer their phones