For (c): create a limit to the amount of votes a user can use, for example, make it impossible to vote on more than 5 items in 24 hours. Story votes, comment upvotes and comment downvotes would all count towards this limit. This would be useless if there is a large number of users who vote up negative comments, and would only work if the problem is caused by a smaller number of users who upvote a lot of frivolous comments and stories.
