Pyrrhus,
I developed a workbook in Excel 97 for distribution a while back and created a custom toolbar with a single button with a blank face. It was only obvious that it was there if you hovered over it. Clicking on it gave you a menu option to enter a password, entering the correct password removed the custom toolbar.
This proved invaluable for maintenance as the workbook evolved after distribution. It wont stop everybody, but then there are usually ways around most things.
I have not used it in some time and have not tested it recently, it could probably do with some tweaking. In case its any use to you or anyone else, here is some of the VBA which you can adapt to suit your own workbook
Sub menuHide()
Set oldMbar = CommandBars.ActiveMenuBar
Set newMbar = CommandBars.Add(Name:="cbar1", Position:=msoBarTop, MenuBar:=True, Temporary:=True)
With newMbar
.Visible = True
.Protection = msoBarNoMove
.Protection = msoBarNoCustomize
End With
Set newMenu = CommandBars("cbar1"

.Controls.Add(Type:=msoControlPopup)
Set ctrl1 = newMenu.CommandBar.Controls.Add(Type:=msoControlButton, Id:=2949)
With ctrl1
.Caption = "TrapDoor"
.TooltipText = "Original Menus"
.Style = msoButtonCaption
.OnAction = "removeCustomMenu"
End With
End Sub
Sub removeCustomMenu()
'
'
Dim Message, Title, Default, MyValue
Workbooks("Book1.xls"

.Activate
Worksheets("Sheet1"

.Activate
Message = " Please enter the password."
Title = "Password Required"
Default = "******"
MyValue = InputBox(Message, Title, Default)
If MyValue = "Correct" Then
Application.CommandBars("cbar1"

.Visible = False
Else
'
End If
End Sub
bandit600