As far as I could tell you can’t do this without modifying the plugin. Here’s what I have done:
comment out all of the actions/filters in wp-content/plugins/share-this/sharethis.php that call the ‘st_add_link’ function. (here they are with their corresponding line-numbers.
159: add_action('the_content', 'st_add_link');
160: add_action('the_content_rss', 'st_add_link');
175: add_filter('the_excerpt', 'st_add_st_add_link', 11);
178: add_filter('get_the_excerpt', 'st_add_st_add_link', 11);
and then in your template-files add this code wherever you’d like the ST icon to show up:
<?php echo st_widget(); ?>
For example, I only wanted the share-this icon to show up when people were actually viewing an article so I put it in my wp-content/themes/current-theme/single.php under the article content
JRGould, thanks for the answer. That did just the trick for me!