Adrian,

The password idea seems like a good one.  You could have a table with
the Users and their authorised passwords.  When they want to check out
an item, they have to select their UserID and name from a combobox, and
their password into a textbox on the form.  The user's authorised
password could be included as a hidden column in the row source of the
User combobox.  The password entered would be recorded in the Borrowings
table (or whatever it's called).  On the After Update event of the
Password control, you could have code like this...
 If Me.Password = Me.User.Column(2) Then
    MsgBox "Take Out is authorised"
 Else
    MsgBox "Invalid password"
 End If
... or some variation on the theme.

Hope this makes sense.  There are actually many approaches you could
take to this from a user interface point of view.  For example, you
might want to actually cancel the entry of the Borrowing record if a
valid password is notr entered.  Or you might want code on the Curent
event of the form, as well as the code suggested above, to toggle the
Caption and/or the colour of a label on the form that says Authorised or
Unauthorised as the case may be. Etc.