If you want to limit the use of styles and themes in a Word document that you and your teammates are collaborating on, there's a tucked away feature that does just that - and it works in both Word 2007 and Word 2010.
First, click the tiny launcher button below the Change Styles button on your Home tab:
This opens the Styles dialog box. Now click the Manage Styles button at the bottom (it's the one on the right just before Options).
Go to the Restrict tab and check both Block Theme or Scheme switching and Block Quick Style Set switching:
Decide whether you want this to apply only to the current document, or new documents as well. Click OK to save your settings.
Be aware that your collaborators can still manually override the fonts. Encourage them to use just the built-in styles, and everything should remain consistent.