Rain billing a cancelled simcard

Rain keeps on deducting the amount after I have cancelled my contract with them. They telling me that the online option of cancelation doesn't work immediately . Why is it there? I have not been using the simcard for months and they keep on billing me.. Decided to cancel and still?? This is infuriating!! And the consultants are not helping at all, they're rude. Rain is a ****.. Don't believe that "pay as you use" they advertise to you!! Why have a cancelation option on their website when it is clearly not working?