One way to reduce spam in the diary domain would be to set a minimum time window between advertising diary entries on the front page. One might have Master diary entries advertised up to once a day, Journey diary entries advertised up to once every two days, etc. As with the articles, members that have not achieved the respect of other members would not be allowed to advertise their views quite as frequently or as strongly. There are systems that deal with negative ratings well. EBay is one of them.
People generally give Positive ratings unless they have something serious to complain about.
The major advantage of this is that sellers are able to easily build a good reputation for themselves if they treat their buyers well. It does, however, produce more reliable sellers. Those with a large number of sales seem to only rarely get a negative rating. Since EBay allows everyone equal rating power, people are more careful about handing out negative ratings. Rather, because Advogato does not have the qualities of EBay's system, Advogato's rating system cannot deal well with negative ratings.