You can make the contents of a field displayed in a Control on a form
uneditable in a number of ways. You can set Enabled to False for the
Control (which will grey it out, but still allow you to read it); You
can display the field value in a calculated control, which will look
normal but be readonly. There are other ways of doing it.
You can also assign a Password to a control (again, the password is
associated with the Control, not, strictly speaking, with the field).
The two obvious ways are either to use the Dirty Event or the
BeforeUpdate Event for the Control to prompt for the password (which
can then be saved so you are not bothered again in that session, if
you like) and to Cancel the Event if the correct password is not
supplied.