Billed for Non-requested Service

When changing my bank account details, in August 2016, I noticed a debit order I was unaware of. I contacted the company in question and I was informed that Optivest provide a Brokers Service for my Medical Aid. I informed them that I did not ever request their service, and that to date i had never received any correspondence or advice from them. This billing was done over a seven year period. I requested immediate cancellation of their service and they continued, regardless of my written request, to bill me for their non-existent service. I have now closed this bank account, so that no further deductions can be done. I have asked in writing, that I be refunded for the years I was billed. To date I have had no response from Optivest.