How to make specific entries always use Text mode and not default Visual mode?

Try it with this in your theme’s functions.php:

add_filter( 'user_can_richedit', 'disable_my_richedit' );
function disable_my_richedit( $default ) {
    global $post;
    if ( $post->ID == 32 ) // change the post ID
        return false;
    return $default;
}

If you need it for more than one post (page) id try this:

add_filter( 'user_can_richedit', 'disable_my_richedit' );
function disable_my_richedit( $default ) {
    global $post;
    if ( in_array( $post->ID, array(32,33) ) ) // change the post ID
        return false;
    return $default;
}