You could create a Word template with all Styles set to 'no proofing' for exam purposes, but it would be an easy matter for anyone to re-set that during the exam - and return it afterwards so no-one would be any the wiser.

Although you could try to delete or rename all the .LEX files, which I believe you'll find in:
C:\Program Files (x86)\Microsoft Office\Office16\PROOF
starting Word (or any other Office program) will then cause Office to launch its setup routines to do a repair.

Ultimately, if you want to prevent the use of spell/grammar checking, tell the users to use WordPad, which comes with Windows and supports formatting but has no spell/grammar checking functions - and, additionally, if you can, don't have Office installed.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]