I and my cofounder used the same idea of users as nodes of a graph for http://oknotizie.alice.it http://oknotizie.alice.it (a system similar to reddit for itali

I and my cofounder used the same idea of users as nodes of a graph for http://oknotizie.alice.it http://oknotizie.alice.it (a system similar to reddit for italian speaking users) in order to indentify groups of spammers and users with very strange behaviour. This information is used in order to decrease the weight of votes in the system for this bad users. Our experience is that while this works very well against spam it does not stop the quality degradation that happens every time the community gets larger because the most active users tend to become friends and stop voting the news just for their quality.