Unauthorised debit orders

I cancelled a contract earlier this year vie telephone. I did not receive a reference number because the lady said that they were not able to provide one. Despite my cancellation, my account was still debited with the debit order. I then called again and received an assurance that the debit order would be stopped and the cancellation recorded. I then also stopped the debit order at the bank. Today I received an sms from a debt collector to claim an outstanding amount on this account. It appears that Vodacom continued to debit me the monthly amount despite the cancellation. Moral of the story - never agree to sign a debit order, and especially not with Vodacom.