This is the way I do it.
It's the only method I've been able to figure out, and there might be a much better way to do it, but it works for me.

1. Edit /etc/cl.selector/php.conf

2. Add these lines right at the top of the 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. In the client's cpanel, change the sendmail path to /bin/true using "Select PHP Version > Switch to PHP Settings"
He will no longer be able to send via php mail

If it works its perfect for me. 

Thanks!!!