You can close your Zoho CRM account permanently by logging in to https://accounts.zoho.com/ (for zoho.com accounts) or https://accounts.zoho.eu/ (for zoho.eu accounts), clicking Preferences and selecting the Close Account option at the bottom.
Important Note:
On closing your account, you will not be able to access any of the Zoho Services.
Before closing, please ensure that you have exported all your data from Zoho CRM and other Zoho services and also, downgrade your account to the Free Edition.
Once you close your account, all the data with the Zoho services will be deleted immediately and cannot be restored in future.