[calorie]

See bolded comments within:

// hook: showthread_getinfo

if (
	$vbulletin->userinfo['usergroupid'] == 1                   // if the user is unregistered or not logged in
		AND                                                // and
	in_array($threadid, array(1, 2, 3))                        // the user is trying to see threadids 1, 2, or 3
)
{
	$vbulletin->bf_ugp_forumpermissions['canviewthreads'] = 0; // then the user gets a no permission page
}

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