If you want to restrict users to your preferred Style set, you will need to apply the editing restrictions and, as I said, you can check the 'automatically update document Styles' option to force a document to revert to the Style definitions in the document's template. You could go even further, with - for example - a macro that removes any direct formatting when a document is opened.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]