de-admin.php

<?php
// Stop Authors from accessing the admin dashboard unless you are an admin.
function prevent_author_access(){
    if( current_user_can( 'author' ) && is_admin() )  {
        // do something here. maybe redirect to homepage
        wp_safe_redirect( get_bloginfo( 'url' ) );
    }
}
add_action( 'admin_init', 'prevent_author_access' );

// Remove the admin bar for all users except for admins.
add_action('after_setup_theme', 'remove_admin_bar');
function remove_admin_bar() {
    if (!current_user_can('administrator') && !is_admin()) {
        show_admin_bar(false);
    }
}

Copy all of this (without the '<?php') and paste it into your themes' functions.php.