This BuddyPress RC2 plugin by Alessandro Raffa removes the default Admin Bar. It’s ready-to-use: just activate it; no options needed.
Plugin homepage: http://www.alessandroraffa.it/en/wordpress-plugins/
Or you can place this code into your wp-config.php file:
define( “BP_DISABLE_ADMIN_BAR’, true );
codex.buddypress.org also has some info on Modifying the BuddyPress admin bar at: http://codex.buddypress.org/how-to-guides/modifying-the-buddypress-admin-bar/