How to remove message output for the WP-PostRatings Plugin?

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.