I found this plugin https://wordpress.org/extend/plugins/remove-buddypress-adminbar/
I also copied and edited the plugin to deactivate the bar just for guests. Here is the code:
<?php
/*
Plugin Name: Remove BuddyPress AdminBar For Guests
Plugin URI: http://blog.netweblogic.com/php/wordpress-php/remove-admin-bar-plugin/
Description: This plugin removes the admin bar completely from the public area of the website just for guests. Can be used either as a normal or MU plugin.
Author: NetWebLogic LLC
Version: 1.2
Author URI: http://blog.netweblogic.com
*/
if ( !function_exists(‘remove_bp_adminbar_guests’) ) {
function remove_bp_adminbar_guests() {
if (!is_user_logged_in()) {
remove_action( ‘wp_footer’, ‘bp_core_admin_bar’, 8 );
remove_action( ‘wp_head’, ‘bp_core_admin_bar_css’, 1 );
function bp_core_override_adminbar_css_perblog() {
?>
<style type=”text/css”>body { padding-top: 0 !important; } #wp-admin-bar { display: none; }</style>
<?php
}
add_action( ‘wp_footer’, ‘bp_core_override_adminbar_css’ );
}
}
}
// Start this plugin once all other plugins are fully loaded
add_action( ‘plugins_loaded’, ‘remove_bp_adminbar_guests’ );
?>
So the bloggers have two options, they can deactivate the bar only for guests or also for the registered users.
