Poor service

I cancelled my sim in September but I am still billed till today. When I call their call centre they say they never saw it but last time in October the person I spoke to confirmed that they received the cancellation. Whats worse is they have now billed me more than even the initial agreed amount. Today I call them they tell me that I will still get billed for December because they will only cancel it today. This is wrong on all levels cos I am constantly billed for something I am not using! why are we being ****med like this!