As far as I know mate (if there anything like normal direct debit's that I deal with at work) then you as the consumer/customer have the right to cancel the direct debit anytime you like.

A direct debit is just a rolling contract for as long as you want it, be careful though as if it's financed via a credit company then it's a whole different matter

Personally i'd ring up and ask to speak to a supervisor or a manager not just any old monkey on the end of the phone HTH