We present a method for jamming a time-division duplex link using a transceiver with a large number of antennas. By utilizing beamforming, a jammer with M antennas can degrade the spectral efficiency of the primary link more than conventional omnidirectional jammers under the same power constraint, or perform equally well with approximately 1/M of the output power. The jammer operates without any prior knowledge of channels to the legitimate transmitters, or the legitimate signals by relying on channel reciprocity.