This is probably documented somewhere but I’m posting it here so I don’t forget (again).

I wanted to give Jetpack’s Carousel module a spin but the built in Themify Pretty Photo lightbox kept getting used instead.

To disable the PrettyPhoto lightbox add these lines to your child theme’s functions.php file.

add_action('wp_enqueue_scripts', 'mh_remove_pretty_photo', 20);
function mh_remove_pretty_photo() {
        wp_dequeue_style( 'pretty-photo' );
        wp_dequeue_script( 'pretty-photo' );
        wp_deregister_script( 'themify-carousel-js' );
}

That will let another lightbox be used instead such as Jetpack’s Carousel. The 20 argument in the add_action() is to make sure my function gets queued after the parent theme function.