Disabling ‘posts’ in Appearance > Menus

We can use the register_post_type_args filter to adjust how the post post type is displayed in the backend with e.g.:

add_filter( 'register_post_type_args', function( $args, $name )
{
    if( 'post' === $name )
    {   
        // $args['show_ui']        = false; // Display the user-interface
        $args['show_in_nav_menus'] = false; // Display for selection in navigation menus
        $args['show_in_menu']      = false; // Display in the admin menu
        $args['show_in_admin_bar'] = false; // Display in the WordPress admin bar
    }
    return $args;
}, 10, 2 );

More info on the parameters here in the Codex.