With pages, there's an option for Full Rights, Create Rights, and View rights. The create rights allow the user to create a new page, but they can't edit existing pages and they cannot delete the page. If the user has access to edit the pages, with Full rights, they will also have access to delete them. In order to remove the right to delete pages, you must remove Full rights, but this also removes their rights to edit pages.
For more information, see our Users & Security Guide.