To Sign Out from all Zoho Apps, click on the Profile link in the top right corner. You will see a drop down with an option to Sign out. Signing out will close all your other open Zoho applications in other tabs in the active browser.
Closing the account will close the Zoho account and terminate access to all the applications of Zoho. You will not have access to any Zoho applications. Only personal users will be able to close their accounts.
Only the admin can delete the the organization users accounts.
In case you have hosted your domain with Zoho or have an active subscription, you need to cancel your subscription, delete the domain, delete the organization and then close the account.
In case you have set up an organization account, and want to change your account to a personal account, or want to join another organizations account, you have to delete the current organization in which you have Super Admin role, before joining the other organization.
The three major steps before you delete the organization are removing users, deleting domains and cancelling any paid subscriptions you are part of. Refer here for instructions to cancel Zoho Mail Subscriptions.
You need to remove all the domains mapped under your organization, before you delete the organization.
The domains registered through Zoho, cannot be deleted from Domains section.