You could try using the BeforeSave event thus :-
'---------------------------------------------------------
Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
If Left(CurDir, 1) = "C" Then
MsgBox ("Cannot save this file to local drive.")
Cancel = True
End If
End Sub
'-------------------------------------------------------------

But I am sure that your users will try other methods - such as simply copying the file across <<grin>>.

....... perhaps similar code in the Open event to stop it being used from there ?

etc. etc. (keeps us in work I suppose).