You can cancel your Knack subscription at any time from your Dashboard at http://builder.knack.com/#delete.
As another option, we do offer a Freeze your account plan which can be done from the "Account" tab in the Knack Dashboard. This will allow you to set your account on standby without losing your app and data for a reduced price.
Note: For security reasons, we cannot delete your account for you. You will need to do this yourself from the Knack Builder.