A secure mechanism is needed to use sensor nodes in real world, since we cannot anticipate attack types. Yu and Guan proposed the dynamic en-route filtering scheme (DEF) to detect and drop false reports as soon as possible in wireless sensor networks. In this scheme, the choice of a threshold value to diffuse keys is important since it represents a trade-off between detection power and overhead. A large threshold value can increase the probability of detecting false reports but it consumes too much energy during the key dissemination phase. Thus, we should choose a threshold value to optimize between the detection power and energy consumption. In this paper we propose a key dissemination limiting method to diffuse keys efficiently. The threshold value in the proposed method is determined by a fuzzy rule-based system by considering the energy level of the network, the number of secret keys in a cluster, and the distance from the base station (BS) to a cluster head. The simulation results show that the proposed method can conserve energy, while it provides sufficient detection power.