How do I remove a customiser option from a parent theme in a child theme?

From the documentation:

To add, remove, or modify any Customizer object, and to access the
Customizer Manager, use the
customize_register
hook:

function themeslug_customize_register( $wp_customize ) {
  // Do stuff with $wp_customize, the WP_Customize_Manager object.
}
add_action( 'customize_register', 'themeslug_customize_register' );

The Customizer Manager provides add_, get_, and remove_ methods for
each Customizer object type; each works with an id. The get_ methods
allow for direct modification of parameters specified when adding a
control.

add_action('customize_register','my_customize_register');
function my_customize_register( $wp_customize ) {
  $wp_customize->add_panel();
  $wp_customize->get_panel();
  $wp_customize->remove_panel();

  $wp_customize->add_section();
  $wp_customize->get_section();
  $wp_customize->remove_section();

  $wp_customize->add_setting();
  $wp_customize->get_setting();
  $wp_customize->remove_setting();

  $wp_customize->add_control();
  $wp_customize->get_control();
  $wp_customize->remove_control();
}

So in your case, you can remove a control using WP_Customize_Manager::remove_control().

Example for the Twenty Nineteen theme:

function my_theme_customize_register( $wp_customize ) {
    // Remove the Colors -> Primary Color option.
    $wp_customize->remove_control( 'primary_color' );
}
add_action( 'customize_register', 'my_theme_customize_register', 11 );