In this paper, we consider physical-layer security for device-to-device (D2D) communications underlaying cellular networks in which cellular users are under threat from multiple eavesdroppers. We propose an access selection scheme for D2D users to protect cellular users against eavesdropping. Specifically, in our proposed scheme, multiple D2D users whose distance to the cellular user is more than the threshold we set will be selected to share the same spectrum as that of the cellular user, and the interference generated by the selected D2D users can be seen as a jamming measure to interrupt the eavesdropping. Then, we consider the secrecy throughput as a standard based on which the security performance of our proposed scheme is quantified, and we derive a closed-form expression for the secrecy throughput of a cellular user with the aid of stochastic geometry. Moreover, by using the optimization method, we prove the existence of optimal threshold and design an iterative algorithm based on the advance-and-retreat method to determine the optimal threshold. Simulation results show that our access selection scheme can effectively improve the physical-layer security of cellular users and that the maximum secrecy throughput can be achieved with the optimal threshold.