It's possible with VBA, but you'd need to password protect your macro as well otherwise it would be easy to 'crack' the protection. Stage 1 would be a worksheet change event macro in the sheet code area of the sheet with the dropdown (right-click the sheet tab name, select View Code, and paste the code into the window that appears on the right of the screen). Adjust the cell reference of the dropdown, and the actual password required.
VBA Code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Range("A1"), Target) Is Nothing Then '<-- *** Change "A1" to your dropdown cell
If Not Target.Value = "Accounting Copy" Then Exit Sub
Application.EnableEvents = False
Application.ScreenUpdating = False
p = Application.InputBox("Enter the password")
If p <> "password" Then Application.Undo '<-- *** Change "password" to your actual password
Application.EnableEvents = True
End If
End Sub
Stage 2 is to password protect your code:
In the VBA Editor select Tools/VBAProject Properties/Protection tab.
Check the Lock project for viewing box
Enter & confirm password (whatever password you like, doesn't have to be the same as in the above code)
Click OK
When you save the file as macro-enabled, the next time you open the file you will be asked for a password to view the code (and as such, see what the Accounting Copy password has been set as).