Stop WP from creating “Sample Page” and “Hello World!” post

If you’re using Multisite

The accepted answer is destructive in that it cancels all other set-up items in the overridden function. A less destructive way to do it for multisite installs is to delete the default content during new blog creation by hooking in to wpmu_new_blog

add_action( 'wpmu_new_blog', 'delete_wordpress_defaults', 100, 1 );

function delete_wordpress_defaults(){

    // 'Hello World!' post
    wp_delete_post( 1, true );

    // 'Sample page' page
    wp_delete_post( 2, true );
}