You can use Review>Restrict Editing, check the option for Limit formatting to a selection of styles, then choose which styles to allow in a document (click on Settings to do this). This turns off the ability to use local formatting to change document appearance. The Formatting Restrictions dialog also has options to block Theme and Quick Style Set switching.
Other than that, users need to be trained in Word, though no one seems to want to spend time or money on training any more.
