To cancel your account simply log into your account and select the My Account link (in the top right of the application). Once you are on the Account page, click the Cancel my account link and follow the instructions.
If you cancel a paid account you will not be charged again, but you are responsible for any charges that have already been incurred for the current billing period. For example, if your billing cycle is on the 10th of every month, and you cancel on the 30th, you will not be eligible a refund for the current month, but you will not be charged again.