Step 1:  Delete User  - Login to your account through https://mail.zoho.com >> click Control Panel (Mail & Docs) >> User Details >> delete the users one by one. Note : The data associated with the user accounts will be lost as explained above in Red.

Step 2: Delete Domain - From Control Panel, click Domains >> Ensure the Mail Hosting envelope icon across the domain name is grayed out to disable Mail Hosting >> Once done, click on the red ( X )  to delete the domain. 

Step 3 : Delete Organization  - (go to) Control Panel >> Dashboard >> Delete Organization.  

Step 4 : To delete / close the account , sign out and sign back into your account through https://accounts.zoho.com/ >> click Close Account.