I am using this script which inserts a time stamp when a user selects an
option from a validated list:

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 2 Then Target.Offset(0, 1).Value = Now()
End Sub

What I need to do is protect the timestamp so a user cannot alter it
manually. When protect the cells, the script will not run because the
protection blocks it. Is there any way to allow the script to enter the
timestamp, but block users from changing the timestamp? Also, is it possible
to make the script run only once per cell? I dont want the timestamp to
change if the user selects another option in the same cell at a later time if
possible.
Thanks