OK, after doing many experiments I though that something there is fishy, so I decided to do it with default TwentyTen theme.
Following instructions at:
http://codex.wordpress.org/The_Loop
I inserted:
<?php if ( is_home() ) {
query_posts($query_string . '&cat=-3');
}
?>
right before:
<?php while ( have_posts() ) : the_post(); ?>
Result: you click "Older posts" and each next page (2,3,4...) shows the same posts.
Is it a bug or I don't understand something? :-/
Thanks for the help!