Just comment out (or change) the following line:
// printf(__('You Had Already Rated This Post. Post ID #%s.', 'wp-postratings'), $post_id);
If you just want to not have the $post_id
, change the line to the following:
_e('You Had Already Rated This Post. Post ID #%s.', 'wp-postratings');
Btw: If you change a plugin and then make an update, all your changes are gone.