8ta / Telkom Mobile
Debiting a clients account after cancelation

I took out a contract with heita after the 2 year period I cancelled it, they went into my account and debited a large amount with making any arrangement or contacting me, when I called the call center the manager Thabang said he could not help me, my question was who gave them authority to debit money off my account without my permission and now the refuse to refund money which belongs to me