I would suggest an OnGotFocus event and the Locked property. You can say something like:

If IsNull(FieldName) Then
   FieldName.Locked = TRUE
Else
   FieldName.Locked = FALSE
End If

When the field gets focus, it will be either locked or unlocked based on whether or not is has data. You'll have to come up with a way around this for Admins who need to be able to edit the value, or just go right to the table.

You can also do a similar trick with entire records in the OnCurrent event.

I've covered these concepts in my classes, but this would make a neat TechHelp video. I'll add it to the list.

Thank you I will try it and hope it works, I will keep a look out for your TechHelp video