My Home or Main Index is a user preference for logging in set in the "Display preferences, number of shown threads, languages, colors...". So you will probally need to run a querry to update your users preference if you want them to go to the main index. If you don't want them to be able to choose you will need to comment out that option. I believe it will be in editdisplay.php, the language file editdisplay.php or editdisplay.tmpl. I'm not sure which.
