How can I prevent users from parking common internet domains such as google.com, hotmail.com, etc ?

Click on the "Tweak Settings" link under "Server Setup". Within the "Domains" section is a check box with the heading:

"Prevent users from parking/adding on common internet domains. (ie hotmail.com, aol.com)"

Tick the check box and apply the changes by clicking on the "Save" button at the bottom of the page.