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In this letter, the issue of security for device-to-device (D2D) underlaying cellular networks is considered. The cellular communication is overheard by randomly distributed eavesdroppers. By sharing the spectrum between D2D users and cellular users, the interference generated by D2D users is used as a source of jamming to confuse the eavesdroppers. We first derive the connection probability of the D2D links and the secrecy outage probability of the cellular link based on stochastic geometry tools. We then propose a joint guard zone and threshold-based access control scheme for the D2D users to maximize the achievable secrecy throughput. Moreover, when only the selection threshold is considered, a closed-form expression of the optimal selection threshold is derived. Simulation results show that improved secrecy throughput can be achieved by allowing the transmission of the D2D users.