Our approach is described for maintaining sustainable cooperative behaviour in agent-based peer-to-peer systems whose environments are organized into agent institutional societies. Instead of expelling uncooperative society members, our approach considers institutional mechanisms that attempt to encourage cooperative behaviour among society members in order to maintain satisfactory operational performance of the institution. This is accomplished by dividing the society into subgroups, each of which is managed by a special monitor agent. The subgroup monitor agents help keep track of member agent reputations and manage the associated tags for each of the agents in their subgroup. Some experimental results are presented describing how these mechanisms operate.