Keep your request simple and straightforward, and include all the necessary details like your name and account number, the cheque number and value, the date of the cheque, the name of its recipient, and your reason for stopping the payment.
Dear Sir or Madam,
I am writing to request that you stop the payment of [cheque number] for the account [account number] in my name. The cheque is for an amount totalling [amount], was issued on [date] and was to be received by [recipient's name]. Regrettably, [your reason for stopping the cheque].
Thank you for assisting me in this matter.
This letter is clear and gets straight to the point, making it easy for the bank to stop your cheque. They will have all the information they require.
If you have the time though, you might want to consider paying a visit to your bank and stopping the cheque in person. Just bring along all the necessary information and inform the teller that you would like to stop payment. It's actually a lot easier to do it this way and there's much less room for human error. You'll be able to communicate what you want and the teller will be able to communicate what they can do for you. This includes offering you confirmation that you're stopping the right cheque and informing you of any other options you might have.
Completing the process in person will also eliminate the potential for your letter to get lost in the shuffle, and assuming that you request some sort of written confirmation that your cheque has been stopped, it will also protect you in the event that something goes wrong on the bank's end. You aren't really afforded that level of security when you cancel a cheque through a letter; in fact, you might find yourself growing anxious over the success of your request.
It's something to think about.