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We introduce transaction rate limiters, new mechanisms that limit (probabilistically) the maximum number of transactions a user of a peer-to-peer system can do in any given period. They can be used to limit the consumption of selfish users and the damage done by malicious users. They complement reputation systems, solving the traitor problem. We give simple distributed algorithms that work over time frames as short as seconds and are very robust: they use no trusted servers and continue to work even when attacked by a large fraction of users colluding. Our algorithms are based on a new primitive we have devised, probably-anonymous queries, which guarantees anonymity with a specified probability.