The best thing to do here would be to use visual basic.
Goto your form properties and double click in the 'oncurrent' field (under events) then click the button with the three dots to the right
You should then get the visualbasic editor appear.
It should show something like
Code:
Option Compare Database
Private Sub Form_Current()
Endsub
Don't touch the top line, however change the rest to read:
Code:
Private Sub Form_Current()
If Isnull([your_control_name_here] )=False Then
[your_control_name_here].Locked=True
End If
End Sub
This should then lock it after someone has been into the field and changed it.
Note; this will also lock it if someone goes in, decides they dont want to type anything and deletes it (makes it zero length string instead).