disable emails on cpanel

if the domain example.com are pointed to your DNS login to WHM and go to Main >> DNS Functions >> Edit MX Entry for the example.com and when Edit the example.com choose “Remote Mail Exchanger” this will Configure server to not accept mail locally and send mail to the lowest MX record.
Double check the example.com MX records to make sure the MX records set to the valid Mail Server. if example.com are NOT pointed to your dns and just use IP forwarding to run on your server you can remove the dns entry for example.com by delete the dns zone for example.com as it not used.
 
http://stackoverflow.com/questions/16709803/disable-emails-on-cpanel