I am not sure, but first You may think of supressing the electric component by passing em wave through dielectric with heavy loss. The idea is, electrical energy consumed during polarisation process. You can materialize this idea if i m not wrong.
Since in photons electric and magnetic energies change their "location" with their frequency, I can´t imagine to suppress one of them without absorbing the photon. A clear separation will be possible in wave guides, where you for example can produce longitudinal electric waves and a circular magnetic field. This techniques is used in wave guides for linear accelerators.