How to stop PHP code running when in a child theme

I understand you can add extra CSS on top of the parent theme, but how do you remove PHP code from functions.php?

I want to remove the Masonry plugin in twentyfourteen theme, and have located the piece of code on line 254.

    if ( is_active_sidebar( 'sidebar-3' ) ) {
    wp_enqueue_script( 'jquery-masonry' );
    }

If I’m editing the parent theme, I can just comment this block out /* */.

But I’m not aware of a way to change this is my child theme.

You can’t actually “remove” PHP code from the parent theme. What you can do is undo things done there.

The counterpart of wp_enqueue_script is wp_dequeue_script.

If you put this in your functions.php it should remove the Masonry Plugin (untested)

add_action( 'wp_print_scripts', 'de_script', 100 );

function de_script() {
    wp_dequeue_script( 'jquery-masonry' );
}

Source: Function Reference/wp dequeue script @ Codex

As I haven’t tested this be aware of the fact that this could have unwanted side effects if the theme relies on the Masonry Plugin.