You want to change default value to unchecked so that user can check that if they want or want to unchecked so that they cannot check and will not get email?

add following code in your theme’s ( child theme’s if any) functions.php

add_filter('fep_get_user_option', function( $value, $option, $default, $userid, $is_default ){
    if( 'allow_emails' == $option && $is_default ){
        $value = 0;
    }
    return $value;
}, 10, 5);