This will leave the portal page open, but still display the warning that the forum is closed. Is that what you wanted?

Unsupported modification alert!

In ubbthreads.php find:
PHP Code
if ($user['USER_MEMBERSHIP_LEVEL'] != "Administrator") {

replace with:
PHP Code
if (($user['USER_MEMBERSHIP_LEVEL'] != "Administrator") && ($ubb != "portal")) {