Add function to Simple Twitter Connect to not Tweet pages

Even better answer that doesn’t involve changing the plugin:

remove_action('transition_post_status','stc_publish_auto_check',10,3);
add_action('transition_post_status','my_custom_publish_rules',10,3);
function my_custom_publish_rules($new, $old, $post) {
  if ($post->post_type == 'page') return;
  else stc_publish_auto_check($new, $old, $post);
}

Put it in a theme’s functions.php.

Side note: Whenever you’re dealing with a function called by an action or filter hook, then you can easily wrap the call in a different function and add your own code to that function instead. remove_action and remove_filter should be two tools you use a lot in your WP arsenal.