They don't cancel expired contract

my top up contract expired around may this year but still they keep on recharging it and demanding payment from me. i went to one of their branches asking for cancellation but still my contract is not canceled. i keep on calling them but all they tell me is that i should give them a month notice. how can i give them a month notice while my contract expired 5 months ago. i even decided to stop paying them because i am paying an expired contract