showing certain level in page parent drop down on admin

Hello folks,

I am working on a website that uses WordPress as a CMS. There is one part of the website that contains approx. 250 subpages. Because the user doesn’t need to be able to add pages underneath these pages I was hoping to be able to hide them from the Page Parent drop down at the right side of the Write Page area.

I’ll assume that there isn’t a plugin that does this but is it possible to modify the code in the edit-page-form.php file so that pages only appear in the list to a certain level? I have been playing with the line of code below to no avail but admittedly, I’m not a programmer.

<?php parent_dropdown($post->post_parent); ?>

Any advice appreciated. Cheers.