It depends of which plan you are currently on. If you are on a monthly plan (flatrate or volume) you can cancel this online. Just click on [My Account] > [Billing] > [Monthly plans & Prepaid]. There you will see your current plan and can cancel it by clicking [Unsubscribe from price plan]
After clicking the button check [Cancel plan] and confirm your selection. You have the option to enter information on why you are cancelling your plan.
With our prepaid plans and free plan you do not need to cancel. If you would still like to delete your account with all saved recipient data please contact support. Make sure to submit your customer number with your request.