unauthorised debit order

I was offered a sim only deal from Vodacom which I originally accepted, when the courier contacted me to deliver I advised them I no longer wanted it and they advised that they would cancel on my behalf. a few days after that Vodacom debited my account, again today a debit order went off my account for a sim card I do not have and received notification that the delivery was received back by them. this is going to ruin my credit score and they will still continue to debit my account for a product I do not have. i need this attended to urgently as the call centre isnt helpful