As a general rule refunds will not be given. However, we're human too, so we will work with users that are in very special circumstances on a case by case basis.
You may cancel at any time. Once you cancel you can continue to use the application until your current billing period ends. You can cancel by clicking the “I understand, please cancel my account” button on the Billing page in Settings. Doing so will permanently delete your account and all your data and by doing so you will forfeit the ability to continue using your account during the remainder of your billing period.