Ray,
Hopefully someone wiser than I will offer something more concrete. But I
think the best way to attack this is to look into using Group Policies.
Within Excel I think you'd have to make massive changes to every potential
workbook to look at all worksheet _Change events and see if anyone has typed
in the \\server entry into a cell. This probably a totally impractical
'solution'.
By definition, the root of a share is shared and unless you control who/what
has access to it via some Group Policy rule, I can't think of another way.
After seeing if someone here doesn't come up with a better idea in say the
next 24 hours or so, you might consider posing the question to a group
dedicated to the Server OS ... this page has a list of all MSFT product
groups:
http://www.microsoft.com/communities/products/default.mspx
