Physical layer security has drawn significant attention in recent years because of its advances in preventing confidential information leakage to eavesdroppers. In this paper, we focus on secure beamformer designs in multi-user multi-input multi-output (MU-MIMO) systems with a multi-antenna eavesdropper attempting to overhear the confidential information of a particular user. Unlike existing works, we aim to exploit inherent multi-user interference to disrupt the reception of the eavesdropper and enhance the security of wireless communications. Particularly, utilizing practical signal-to-interference-noise (SINR) metrics, the objective of secure beamformer design is to minimize the eavesdropper's SINR, while providing legitimate users with required pre-specified SINRs to guarantee quality of service (QoS).We propose secure beamformer designs for the downlink transmission situation, under the conditions of known and unknown eavesdropper's channels, respectively. With the proposed secure beamformers, the multi-user signal behaves like artificial noise or distributed friendly jamming, which can further degrade the reception of the eavesdropper. Extensive simulation studies confirm our analytical performance predictions and illustrate the advantages of the proposed beamformer designs for secure MU-MIMO transmissions.