I use Comment Closer to close comments on old posts to reduce spam on my blog. I wanted visitors to know immediately if a post was no longer accepting comments, and give them alternative means of contacting me about the post. You can see it in action in these two old posts: post with comments, post with no comment. Here is how I did this in Drupal 6:

• Posts with no comment

  Use the following in node.tpl.php file in your Drupal theme:

  
  <?php if (($page != 0) && isset($node->comment_count) && $node->comment_count < 1) { ?>	
  	<?php if($node->comment == 0 || $node->comment == 1) { ?>
  	// Enter HTML content you want to render for posts
  	// that are closed with no comment.
  	<?php else { ?>
  	// Enter HTML content you want to render for posts
  	// that are open for new comments but have none.
  	<?php } 
  } ?> 

• Posts with comments

  In comment-wrapper.tpl.php, use the following:

  
  <?php if($node->comment == 0 || $node->comment == 1) { ?>
  	// Enter HTML content you want to render to inform the visitor
  	// that new comments are no longer accepted.
  <?php } ?>  

We need to edit both comment-wrapper.tpl.php and node.tpl.php as comment-wrapper.tpl.php (despite its name) will NOT be rendered if a node has no comments.