bad service for unauthorised debit order

I had funeral cover with FNB cancelled it last year August/September but they still deducted my money till last month. sent them email with my bank statement still looks like that they refusing to refund my. I would try disputing the debit order but following month they would three times and revoke access of disputing their debit order, Is it because am using their bank or what, WHAT gives them the right to debit without the customer's consent. Sent numerous emails but till this date nothing.