Easy.
Say this is your document:
This text needs to be immutable.Highlight the first section you need to be unalterable:
This text does not.
This text needs to be immutable.Then go to Insert / Section. In the "New Section" field, change "Section1" (if you so desire) and rename it to something like "Immutable 1". Then tick the "Protect" box, and finally click on Insert.
This text does not.
That line of text is now unchangeable.
And if you're an emacs fan, emacs can do this also by changing the Properties of the text to read-only.
Originally published at:
https://kentwest.blogspot.com/2019/03/immutable-text-sections-in-libreoffice.html