Since everyone will have weighted voting power, all you will have to do anyway to game the system is upvote anything with a lot of votes. Steadily build up your karma with this bandwagon method, and then bam spring some ad-infested blog spam upon everyone.
This can be accounted for by giving more weight to those who vote for a story early in its accumulation of points. The biggest voting power boost is given to someone who gives a story its first point.
