@swolfe, this can be done with a small modification to the source.
In wp-includes/template-functions-author.php, around line 155 you’ll find the start of the list_authors function. Locate the $query:
$query = "SELECT ID, user_nickname, user_firstname, user_lastname, user_nicename from $wpdb->users " . ($exclude_admin ? "WHERE user_login <> 'admin' " : '') . "ORDER BY user_nickname";
and change it to:
$query = "SELECT ID, user_nickname, user_firstname, user_lastname, user_nicename from $wpdb->users " . ($exclude_admin ? "WHERE user_login <> 'admin' " : '') . "AND user_level > 0 ORDER BY user_nickname";
A parameter to set this may be useful to a number of folks. I’ll look at submitting the idea to the devs…
Note: Back up any source files before editing them, and comment your changes for future reference.
(A similar hack could be performed to exclude specific users, but then we’re getting into an area of possibly having to regularly maintain changes in the source. Blech.)