How to restrict template types that can be inserted under a Sitecore content node

Restrict item types that can be added to content tree item.
How to restrict template types that can be inserted under a page (using insert options on Standard values)

Step-by-step guide
 1. Select "__Standard Values" of the page template (that is used to create the page)
 2. Select "Configure" from the toolbar
 3. Select "Assign" from the toolbar
 4. Select template type that will be allowed (eg.  "Special Offer" from /sitecore/templates/Project/abc/Content Types/) and add to the "Selected". Remove everything else from the list

5. Press ok to save
6. If you try to create an item under the page (created using the page template)., Sitecore will allow you to create only item of "allowed type"