Kill off lowest hierachy in parent site select box?

[Bolkonskij]

I have a page with a lot of sub pages (think: close to 1000). Whenever I create a new page and choose select box for "Parent page", that brings up a LOT of entries. It actually makes it quite hard to find the respective category I want to append this page to as a child.

So here's my question: Where (file) and how do I eliminate the display of child pages in the parent page select box?

Here's a quick sketch to show what I'm talking about.

Any ideas?