I am not sure why setting the security didn't work (you should be able to create a new role, say testonly, and allow only users with that role to see the test page, and of course never assign any users to that role).
But as far as the test page reappearing goes, I think I know why that happened. The test page is defined at jboss-portal.sar/portal-sample.war/WEB-INF/default-object.xml. As long as that file exists, every time you start the portal, that file is read and is used to update the portal settings. Which means that if you deleted the test page, it will re-create the test page. To completely get rid of the test page, remove this file.