using something like:
http://mvps.org/access/api/api0008.htm (Get User Login Name)
you can easily get the current user's windows login (assuming everyone has
there own).
Then, in the Form_Open event, you could disable/enable the field/control
depending on the user login.
This assumes that you've set things up so that users can only modify data
via forms and that they can't open tables directly. If they can, look into
UserLevel security.
This also wouldn't prevent anyone from executing ADO/DAO/SQL code in another
app (say Excel) that would update that file-table-field, but it might be
sufficient for your purposes.
HTH,