Provide the necessary information to the company so it can easily access your information and cancel your policy. If the thought of writing a letter puts you off because you feel you lack writing skill, realize that your purpose here is more about providing the facts than flowing rhetoric.
Set forth your name and address, insurance policy number, phone number, insurance agency's name and address and the date you wish to terminate your coverage. That information is absolutely necessary; optional information is your reason for canceling, policy effective dates and the company to which you are switching and its rate you will pay.
Provide a subject line reading "Re: Cancellation of Insurance Policy #_." The letter can then read something like, "Please take this letter as a formal request to cancel my insurance policy and stop all charges to my credit/debit/bank account from your agency. Please make this cancellation effective as of (date). I also request written confirmation of this cancellation, along with a refund for unused premiums, within 30 days of this action."