If the default blog is the ALL blog (this is default at setup) then I don't know a solution to your question. To my knowledge t is not possible to exclude posts from the ALL blog.

But you could try this instead:
- Make a new blog and set it as your default blog
- In ../blogs/conf/ change the file _admin.php where it reads:

/**
 * Cross posting
 *
 * Possible values:
 *   - -1 if you don't want to use categories at all
 *   - 0 if you want users to post to a single category only
 *   - 1 if you want to be able to cross-post among multiple categories
 *   - 2 if you want to be able to cross-post among multiple blogs/categories
 *   - 3 if you want to be able to change main cat among blogs (which will move the
 *       posts from one blog to another; use with caution)
 *
 * @global int $allow_cross_posting
 */
$allow_cross_posting = 1;

Set the value $allow_cross_posting = 1; to $allow_cross_posting = 2.
- Now every poster can decide whether his post should appear on the frontpage or not.

Don't try this if tou have a lot of post on your blog already; old posts wil not automatically be included in your new frontpage.