Code in a functions.php (or any file loaded by that file) should not run automatically, eg. when the file is included. All code should be bound to hooks.
That means, you don’t need a special action in the parent theme. The child theme can turn off all parent theme callbacks with remove_action() and remove_filter().

But you can make that easier for child theme authors by adding a custom action after you have registered the callbacks.

Example

Parent theme

namespace ParentTheme;
// register callbacks
add_action( \'after_setup_theme\', __NAMESPACE__ . \'\\setup_custom_background\' );
add_action( \'after_setup_theme\', __NAMESPACE__ . \'\\setup_custom_header\' );
add_filter( \'body_class\',        __NAMESPACE__ . \'\\enhance_body_class\' );

// provide an entry point
do_action( \'parent_theme_loaded\', __NAMESPACE__ );

Child theme

add_action( \'parent_theme_loaded\', function( $namespace )
{
    remove_action( \'after_setup_theme\', $namespace . \'\\setup_custom_header\' );
});

See also this similar example for outdated PHP versions.

If your parent theme ignores the principle of separation of concerns and mixes everything in just one or two setup functions, use a later priority and overwrite those calls:

Parent theme

add_action( \'after_setup_theme\', \'some_setup_function\' ); // priority = 10

Child theme

add_action( \'after_setup_theme\', \'fix_theme_setup\', 11 ); // priority = 11

function fix_theme_setup()
{
    remove_theme_support( \'post-formats\' );
    // more changes here
}