When a new MemberGate site is installed, the forum preview messages are defaulted to appear on the right navigation panel of your site.
To hide it until you have some posts to your forum (or to hide it indefinitively):
1. Choose 'Edit Category' and select 'Discussion Forum' from the drop down menu
2. In the form field 'Show Category on Right Nav Panels' choose 'Don't Show on Right Panel on Any Pages'
When you are ready to have the preview on your portal pages, change the Discussion Forum Category to show on the navigation panel of your choosing.