Experiments suggest that this MRU is held in Word's "Data" key in the
registry at

HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Word\Data

probably in the "Settings" value. I don't know of a way to clear the list
other than deleting or renaming that value, but the values in Data contain a
lot of configuration information so it is unwise to do that unless you are
really willing to discard all that information.

Peter Jamieson