Repeated failure to stop debit order

I had a contract with On Air which I cancelled after the 24month period. They simply refused to terminate the debit order. It will stop for a month or two after my request to my bankers and then it just re-appears again. When I called them for the 100th time, they referred me to Deals Direct indicating that they are actually at fault not to cancel the debit order. Deals Direct then inform me that I must give yet ANOTHER one month's notice......................