More karma, more posting; less karma, less posting. Everyone starts every month/week/day with only so much, no roll-over per timeframe. Modify it so that 'popular' threads cost more to post in. You can give karma to others too via the upvote and take it away with a downvote, but still no roll-over. Troll/shill accounts would still get upped around en masse, but less so and it would be 'easier' for mods to tell. (I'm sure you can model this without too much effort vis a vi prisoner's dilemma). You'd have to pick and choose which to comment in. Posting content would work similarly, but a slight mod to the cost to posting.