Put a field on your document called "Status".
Put the following code into the QueryModeChange event on your form:
Sub Querymodechange(Source As Notesuidocument, Continue As Variant)
If not source.editMode Then
If Source.Document.Status(0) = "Closed" Then
Messagebox "You may not edit a closed document."
continue = False
End If
End If
End Sub
Write some code into your save button or into the status field that updates the status field to closed when the document is saved.
When a user opens the document and tries to edit it the QueryModeChange event will fire, check the status field and not allow the mode change to continue if the status of the document is closed.
hope this helps!