FNB allowing unauthorized debit order to go off bank account

I wrote to FNB in December that there is debit order online subscription that i did not authorize that went off my account. I was asked to show them proof that i have informed the company that is debiting my account that i have not authorized this debit order - i showed them. It's now June an this company is still debiting my account. I am a private wealth client and i have also written to the private banker -no response. I have had this account for more than 30 years and i have to ask them to close it since they cannot stop unauthorized debit order coming off my account on their watch.