You can cancel your contract yourself in the dashboard (menu "My Package" → Cancel contract). The notice period is 1 day before the end of the paid period, and annual contracts run until the end of the year (T&Cs § 7). You confirm the cancellation twice. A cancellation can be withdrawn, and you can come back at any time with a new package.
Your account itself remains active after your contract ends, among other things because of your royalties. If you would like to have your account deleted completely, send us a ticket in Support. We'll handle this on a case-by-case basis, because invoices and billing data have to be retained.