Wrongful Debt Collection

When my contract was about to expire with mtn, mtn called to offer me a new one or some sort of data deal. I refused and said I would not proceed with any of their deals upon expiration. Mtn continued to debit my account even after the contract had expired, I had to reverse the debit orders every month (mind you using my own petrol and time). Today I am being called by debt collectors from mtn forcing me to pay money I should have not been paying in the first place. What nonsense is this?