1. Logged into your server via SSH and Edit /etc/cl.selector/php.conf file.
Directive = sendmail_path
Default = /usr/sbin/sendmail -t -i
Type = list
Range = /usr/sbin/sendmail -t -i,/bin/true
Comment = Switch to /bin/true to disable php mail
3. Then, logged into client's cpanel, change the sendmail path to /bin/true using "Select PHP Version > Switch to PHP Settings".