Have you applied editing restrictions via Developer|Restrict Editing>Formatting restrictions? You can use that to limit formatting to a predefined set of Styles.

Also, if you haven't already done it, you could go to Developer|Document Template>Templates and check the 'automatically update document Styles' option. Whenever the document is reopened, that will revert any Style changes a miscreant might make to the definitions found in the document's template.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]