I'm not sure how editability is stored on an item but I was writing IOU's the other day and considering using them as a written form of currency backed by a reputable individual (gold standard hehe.) But I realized that another PC could just edit the note and make it say whatever they wanted.
I got around this issue by using a LORE bird message since those are uneditable. However, they're also 2x2 making carrying serveral around as currency rather onerous.
So, It'd be great if there was a quill command to "finish" a note making it uneditable. IC wise, you wouldn't be able to remove the ink once you put it on (easily and without leaving a trace) however you need to be able to do that while editing it IG incase you make a typo or something.
I suppose another way could be to make "indellible ink" that is not eraseable, have some sort of invisible tags around it and make the delete and erase commands ignore characters between the tags. If you mess up writing one you just have to toss it and start again.