remove_theme_support()

function remove_theme_support( $feature ) {
	// Do not remove internal registrations that are not used directly by themes.
	if ( in_array( $feature, array( 'editor-style', 'widgets', 'menus' ), true ) ) {
		return false;
	}

	return _remove_theme_support( $feature );
}

// in your Child Theme's functions.php    

// Use the after_setup_theme hook with a priority of 11 to load after the
// parent theme, which will fire on the default priority of 10
add_action( 'after_setup_theme', 'remove_featured_images_from_child_theme', 11 ); 

function remove_featured_images_from_child_theme() {

    // This will remove support for post thumbnails on ALL Post Types
    remove_theme_support( 'post-thumbnails' );

    // Add this line in to re-enable support for just Posts
    add_theme_support( 'post-thumbnails', array( 'post' ) );
}