Could be a mod but first steps are do not enable the portal page.
This may work if you don't enable the portal page.
Control Panel » Portal Settings
turn off
Enable the Main Portal Index page?

goto control panel> group management.
Select site for guests.
Turn everything off meaning set to 0.
Then again goto control panel>group management select forums permissions.
Again set all permissions in all forums to 0.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers