You can prevent users from modifying shared pages by making a shared page read-only or mandatory.
Read-only prevents a user from adding portlets, or making changes to the content, layout or name of the shared page. The user, however, can add the page using the add page list and delete the page from the portal. Read-only pages can be added to the portal only once.
Mandatory pages are always present for all users. The restrictions are the same as read-only pages, except the user cannot add a mandatory shared page using the add page list or delete the page from the portal.
When you update a read-only or mandatory shared page, it automatically updates the next time the user logs on or refreshes the page.
When a user is removed from a role, a read-only shared page remains in the portal until the user deletes it. However, a mandatory shared page is removed from the portal the next time the user logs in or the page refreshes.