From LibreOffice Help
You can protect, hide, and convert sections to normal text in your document.
- Choose Format - Sections.
- In the Section list, click the section you want to modify. You can press Ctrl+A to select all sections in the list, and you can Shift+click or Ctrl+click to select some sections.
- Do one of the following:
- To convert a section into normal text, click Remove.
- To make a section read-only, select the Protected check box in the Write Protection area.
- To hide a section, select the Hide check box in the Hide area.