Dear (insert last name)
Re: Mention here your serial number, or invoice number, or your membership number, whichever applies to your cancellation.
Paragraph 1: Introduction. In this paragraph you need to explain that you intend to cancel your subscription or contract, and here you should apply the date the contract or subscription started and when you wish it to end. You should also ask for written confirmation that your contract has been cancelled, and if need be you can include payment for anything that is outstanding on your balance.
Paragraph 2: Here you should reiterate your desire for the contract or subscription to end and that you have no desire to pay anymore instalments following your final payment which is enclosed. You should also ask for their prompt attention to the matter.