Unauthorised Debit Orders

Bought a vehicle from WBC a few years ago, this came with a 3-year Netstar Contract included in the financing. This vehicle was sold over a year ago, and Netstar was telephonically informed of this, and asked to cancel the contract. Now, the 3-year contract period has come to an end, which means it usually goes on to be a month-to-month contract. However, we requested the contract be cancelled over a year ago. Last week I received an email with an Invoice. I contacted Netstar to ensure the cancellation was done. I was told to send an email to their cancellations email address, we did. Yet they still debited our account and took our money over the weekend. This is unacceptable and unprofessional for such a big company to let things like this happen.