There are a few WordPress plugins that add text to the main content, such as "Related Entries" and "Similar Posts". (I prefer the latter due it being updated frequently.)

Other types of of plugins are available, but for my example I want to use Similar Posts. The proper way to include the plugin, in addition to the initial installation, is to add a line of code to single.php and/or index.php, similar to this:

<?php if (function_exists('similar_posts')) similar_posts(); ?>

By including the code in the theme, the plugin affects every post. How do you prevent it from affecting a specific post? Change that line to:

<?php if (function_exists('similar_posts') && !stristr($post->post_content, '<!– No Similar Posts –>')) similar_posts(); ?>

When you write or edit a post, you would simply add   <!– No Similar Posts –> to the end of the post, which would basically tell the post to ignore that plugin.

This kind of hack will work for any plugin that adds text onto the content. Sometimes it's easier to do it this way than to try to use the options available with the plugins.