As Chris mentioned in the other thread, Jetpack should already delete the old spam feedback for you, via the wp_delete_post() function that deletes posts and all associated meta data (ref).
Unfortunately I couldn't test it myself as I'm a bit of a maniac and always clear spam myself manually. I don't have any old spam on any of my test sites. :(
You mention deleting them manually: do you use the "Empty Spam" button under the Feedback menu in your dashboard, or do you actually go into your database and delete each row manually?
