Define Pages are used in front user interface / website and display static information. These pages can be easy changed by the owner of the website. Changing them do not require IT knowledge.

1) How to delete / disable pages from information box / information option menu?

Go to adiministartive interface
Go to Configuration -> Define Page Status

2) How to delete / disable from administrative area ?
It is about drop down list from : Tools -> Define Page Editor
Delete phisicaly files located under:
- includes/english/html_includes/
and
- includes/english/html_includes/YOUR_TEMPLATE_NAME (in case you have created this folder)
for example delete define_page_2,php

3) How to add a new page into administrative area ?
It is about drop down list from : Tools -> Define Page Editor
Add a new page for example my_new_defined_page.php in BOTH floders:
- includes/english/html_includes/
and
- includes/english/html_includes/YOUR_TEMPLATE_NAME (in case you have created this folder)