Forced to pay for service no longer wanted

I called the call centre to cancel my DSTV subscription as it is the last day of the month and dstv is paid a month in advance. I asked that my subscription be cancelled as of tomorrow and they refused stating that because my debit order is on the 26th of every month, they can only cancel on yhe 26th. Why should I as a consumer be forced to pay for a sevice I no longer want. This is also the 3rd time that I am asking for the service to be cancelled.