contract expired in July 2015

I send a completed cancellation form to the cancellation dep, who then send me an email promising to contact me soon yet nothing happened then I made follow ups then received another apology email from cancellations yet to no avail. 3 team leaders from the call centre also promised that they have escalated the matter to the relevant department still no service received but I still receive the monthly bill yet I have been trying to cancel the contract from 7th of July. I don't understand why they expect me to pay while I'm not receiving any service or joy. I was told about 21 days waiting period while making follow up for the 4th time yet their 21 days has past. I spoke to 5 team leaders and 7 call centre agents yet the matter is not resolved to date.