I've always used -1 to be true in VSTO and in a few places it seems to matter compared to using 1 for true.

I needed to disable spell check on all endnotes. The answer of using NoProofing worked, but I later found an easier way to change the "Endnote Text" style to add NoProofing as -1.

In my code this is:

wordDocument.Styles[Word.WdBuiltinStyle.wdStyleEndnoteText].NoProofing = -1;

This can be done with other styles as well.