Mark all of the text to not check spelling or grammar. You can do this by Ctrl+A and then under the Review tab under Language > Proofing language, make sure the box to not check spelling or grammar is checked.




You can also do this with a character style for individual words or passages. Such a style is described toward the end of this Article:
Proofing Language Keeps Changing - Solutions - How can I keep my proofing language from changing?

Charles, thanks. Strangely, that Language/Proofing language option was already set to Not spell check, as you suggested. I will mark this query as solved for the time being but will keep an eye on Word making unwelcome automatic corrections and try to track down why it's doing that, via Language options.