enable email but disable email change.

I am creating a custom authentication plugin.
The user email is loaded from an external source and it is stored in the
mediawiki DB.

The problem is that if  a user go to his preferences page, it will have the
chance to change his email address.

Looking in specialPreferences.php I have found this:

        if ( $wgEnableEmail ) {

            $moreEmail = '';
            if ($wgEnableUserEmail) {
                $emf = wfMsg( 'allowemail' );
                $disabled = $disableEmailPrefs ? ' disabled="disabled"' :
'';
                $moreEmail =
                "<input type='checkbox' $emfc $disabled value='1'
name='wpEmailFlag' id='wpEmailFlag' /> <label
for='wpEmailFlag'>$emf</label>";
            }

            $wgOut->addHTML(
                $this->tableRow( Xml::element( 'h2', null, wfMsg( 'email' )
) ) .
                $this->tableRow(
                    $emailauthenticated.
                    $enotifrevealaddr.
                    $enotifwatchlistpages.
                    $enotifusertalkpages.