We present a method to control the polarization of guided waves in a mm-wave dielectric rod waveguide. A pair of optical posts is situated on either side of the waveguide, and this markedly raises the cutoff of the undesired mode. The isolation is 20 dB over a frequency span from (55-87) GHz. This can be viewed as a proof-of-concept for reconfigurable, general-purpose polarization control and dispersion engineering in mm-wave and terahertz-range dielectric waveguides.