Users should not be able to add a particular domain ( say domain.com ) as an add-on domain or a parked domain.
This can be easily accomplished on a cPanel server by the following steps :
1. Add the domain ( domain.com ) to the file /usr/local/cpanel/etc/commondomains
2. Then, ensure that the following Tweak Setting (WHM -> Server Configuration -> Tweak Settings) is enabled:
Prevent users from parking/adding on common internet domains. (i.e. hotmail.com, aol.com)