One way to acheive this is to create a filter on the_content();
This will completely stop the <!--nextpage--> shortcode from working wherever the_content(); is called in your theme – i.e. on all posts
You can add the filter by adding this to your functions file:
function remove_page_144242($content){
global $post;
// Accesses the post content while unformatted
$content = $post->post_content;
// Find the nextpage shortcode
$find = '<!--nextpage-->';
//Replace it with nothing
$replace="";
// Perform the replace
$content = str_ireplace($find,$replace,$content);
// Return the final content
return $content;
}
add_filter( 'the_content', 'remove_page_144242', 1);
If you’re looking for a solution on a single post basis or on a template then you can do something similar. By first pulling back the unformatted content and doing a replace. Something like this could possibly be placed in template file specifically for a post
// (While in the loop)
// Make sure we can access $post
global $post;
// Access the post content unformatted
$content = $post->post_content;
// Find the nextpage shortcode
$find = '<!--nextpage-->';
//Replace it with nothing
$replace="";
// Perform the replace
$content = str_ireplace($find,$replace,$content);
// Echo the final result
echo $content;
EDIT: Apologies the last approach will not perform actions such as shortcodes in the content