There is two main processes that run email on a cPanel server.

1. EXIM - Used for email clients, pop3, and imap.
2. PHP - Used for website forms mostly.

To disable exim server wide, you just need to go to the services section in your WHM, and un click: exim, imap, mailman, pop3, and spam assassin.

To disable PHP mail server wide when using PHPFPM, you have to go into the root of the server and do this: 

Add /var/cpanel/ApachePHPFPM directory if it does not already exist.
Create the /system_pool_defaults.yaml file.

In the new /system_pool_defaults.yaml add this code: 

php_admin_value_disable_functions: exec,mail,passthru,shell_exec,system