Naturally, we are always sorry to see a user go !
However, if you wish to unsubscribe and delete your account data, simply go to your user profile (e.g. by clicking your name, top left), click on the Plan & Quotas tab, and click on the 'Delete Account' link at the bottom (in red).
After a confirmation window, your account and all associated data will be permanently deleted.
Rather than deleting your account, you can also keep a basic account that allows you to keep your project data and restart a subscription later if needed.