If you'd like to deactivate your account, you can do so by going to your account settings page and clicking the "Deactivate my account" button. Deactivating your account is a less permanent way of shutting your account down, and locks down your account without actually deleting its data. This way, if you change your mind later, you'll always have the option to write to us at firstname.lastname@example.org and request that we reactivate the account for you.
If you'd like a more permanent solution, you can fully delete your account, and all the data associated with it, from your data export page. If you do this, however, your account will not be recoverable, and all your progress will be permanently lost.