'm a little late to this party, but it seems to me that the solution to this problem would be to just downvote posts that buy votes when they are 5 or 6 days old rather than to target the services that sell votes.
Even if all the services on this list implement the change, some new ones will pop up to serve the market who wants to buy votes on their posts right before payout.
I would think it would be better to penalize the posts that buy votes at the last minute in order to curb this behavior.