Table security

I have an Access 2003 database. On a form a user can select values by
means of a combobox. If the user enters a value that is not present in
the underlying table, the 'on not in list' event asks the user if he/
she would like to add the new entry to the table. If the user answers
yes, a password form will be displayed asking the user for a password.
If the password is correct, the new entry will be added to the table.
However, the user could also bypass this by simply select the table
and add the new entry manually. How do I prevent the user from doing
that? Is there a way to force the user to always add new entries to a
table throught the form?