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Content pollution is a de facto problem in several currently popular Peer-to-Peer file sharing systems. Previously proposed strategies to fight content pollution include a peer reputation system named Scrubber as well as the alternative Credence object reputation system. This paper builds on previous work into three directions. First, it evaluates the cost-effectiveness of Scrubber and Credence in reducing pollution dissemination when polluters make use of collusion and Sybil attacks. Second, it modifies the Scrubber system to increase its effectiveness, in particular under collusion. Finally, it proposes a hybrid peer and object reputation system that combines the benefits of both strategies. We performed an extensive evaluation of all three systems, for various configurations and pollution mechanisms, as well as collusion and Sybil attacks. Our results show that the new hybrid system is much more effective and robust to malicious actions than any individual strategy, even in very uncooperative and unreliable peer communities.