Is there a warning on submission that says something like "Please don't ask your friends to upvote your submissions. If you do, your submission may be penalized. The best products stand on their own merits."? If that's the case, then why do you Favorite Tweets from companies begging for upvotes instead of warning them that their behavior is unacceptable?
I personally have received spam asking for Product Hunt upvotes ( http://i.imgur.com/B5hLdYG.png ), and you of all people should be interested in not supporting that.