If you must insist...

Most people will make their logo or banner redirect back to the portal page. If they don't want to go that option, then they will add a link, most often reading "home" to the overall_header.tpl file which controls the upper portion of your theme. With that being said there are a couple of options to do this, 1. making your logo or banner a clickable that will open in the same window (the self) command or (target command) opening in a new window. They direct it back to the portal.php file so when visitors click the link or image it will take them to the portal instead of the index. Now seeing that you don't have access to this file, you can't change it. Now seeing that Forumer.com along with other free forum hosts use some type of multi-forums script (sebs being the most popular) changes done to your forum will have to be made to all forums. Not all people would want this redirect.

Now if you really want to accomplish this, you could make a locked forum with the title stating whatever and make the link take visitors back to the portal. Again, you will have to make a forum to do this, which in some cases appears to be a bit tacky. I hope this helps.