If your Linode is powered off you will still be billed for it. Linode maintains your saved data and reserves your ability to use other resources like RAM and network capacity, even when your Linode is powered off. You will also be billed for any other active Linode service, such as Longview Pro or extra IP addresses.
If you want to stop being billed for a particular Linode service, you need to remove it from your account entirely.