[darkray16]

So when a user signs up on the site and creates a new blog site, I want to load my theme as the default theme for newly registered users. I also do not want the user to have the ability change the theme.

How would I go about this?

[@mercime]

Add this to your wp-config.php file
define(‘WP_DEFAULT_THEME’, ‘theme-folder-name’);
from this tutorial

As for changing themes, as long as you activate only your preferred default theme sitewide in Network Admin, the only change they can make is to twentyten which needs to be there just in case.