  Many protocols for distributed and peer-to-peer systems have the feature that
nodes will stop providing service for others once they have received a certain
amount of service. Examples include BitTorent's unchoking policy, BAR Gossip's
balanced exchanges, and threshold strategies in scrip systems. An attacker can
exploit this by providing service in a targeted way to prevent chosen nodes
from providing service. While such attacks cannot be prevented, we discuss
techniques that can be used to limit the damage they do. These techniques
presume that a certain number of processes will follow the recommended
protocol, even if they could do better by ``gaming'' the system.
