Exclude posts by year

To exclude posts, using the WordPress Default theme for example, in the wp-content/themes/default/index.php file, just before the line:
<?php if (have_posts()) : ?>

put this:

<?php
  function filter_where($where = '') {
    //retrieve posts dated Jan 1, 2006 and newer
    $where .= " AND post_date >= '2006-01-01'";
    return $where;
  }
add_filter('posts_where', 'filter_where');
query_posts($query_string);
?>

The query_posts() article explains the arguments in detail.

Related:
Stepping Into Template Tags
Stepping Into Templates
Template Hierarchy