How do you determine who is actually entering the data? Do you have some form of login system?
You can "password protect" the field using the Enter or GotFocus event of that field:
Dim sInput as String
sInput = InputBox("Enter your password:")
If sInput <> "TomsPassword" Then
Msgbox "You cannot enter data here"
Me.SomeOtherControl.SetFocus
End If
You'd have to do this for each notes box.
I also have to question the logic of having a field tied to a person. This is certainly not a normalized structure (what happens when Tom leaves, or when you need to add another user). I would certainly review the database structure to insure you've normalized it before continuing forward.