Contract cancellation

I cancelled my contract end of February by sending the appropriate cancellation letter to the email provided by the customer care representative, I received a notification that the mail has been received and will be actioned. I paid for the month notice and thereafter accepted that the contract is cancelled. I therefor stopped the debit order as I did everything they requested for the cancellation. I now started getting invoices in the mail for June, July? I have replaced the alarm system in May with a new system and was informed the old system was not working at all, so in fact I was being charged for services and armed response without a functional alarm system? I am now receiving no service from Chubb, however I am be billed despite the fact that I cancelled my services? Please advise what the way forward is?