Foolproof is hardly possible but the following might help
Save the document as a macro enabled template including the following code
Code:
Option Explicit
Sub FileSave()
If ActiveDocument.Saved = True Then
ActiveDocument.Save
Else
With Dialogs(wdDialogFileSaveAs)
.Password = "Password"
.Show
End With
End If
End Sub
Sub FileSaveAs()
With Dialogs(wdDialogFileSaveAs)
.Password = "Password"
.Show
End With
End Sub
Create new documents from it and when the user saves or uses saveas the code will add the password 'Password' to the document.