Is it possible to stop WP from creating the “Sample Page” and “Hello World!” post when creating a new blog?

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 );
}