In this paper, we consider a wireless network setting where a base station (BS) employs a single unmanned aerial vehicle (UAV) mobile relay to disseminate information to multiple users in the presence of multiple adversaries. The BS, which is on the ground, has no direct link to the users or the adversaries, who are also on the ground. We optimize the joint transmit power of the BS and the UAV, and the UAV trajectory. We introduce the information causality constraint and maximize the average worst-case secrecy rate in the presence of the adversaries. The formulated average worst-case secrecy rate optimization problem is not convex and is solved sub-optimally. First, we optimize the transmit power of the BS and the UAV under a given UAV trajectory. Then, we optimize the UAV trajectory under the sub-optimal UAV and BS transmit power. An efficient algorithm solves the average worst-case secrecy rate maximization problem iteratively until it converges. Finally, simulation results are provided, which demonstrate the correspondence of the UAV optimal track and transmit power allocation to what is suggested by the previous theoretic results.