The billing tab is very simple. It shows you what your current plan is and gives you the option to view addons / upgrades. There is also a black button to cancel your plan. Click this button if you want to delete your account.
If you click the Cancel Plan button there will be a popup asking you if you are sure and explaining that your account will be immediately terminated. Only do this if you want to permanently remove your account and data. We cannot get it back for you afterwards.