We propose a switching method for optical beaming generated from a metal slit surrounded by surface gratings. The principle of the method is based on the interference of diffracted surface plasmon polaritons from the gratings which are controlled by the relative phases of two oblique incident beams that are illuminated on the metal slit. By adjusting the relative position of the interference pattern of the incident beams with respect to the metal slit, beaming from the proposed structure can be switched from the on- to the off-mode by virtue of the change in the symmetry of the generated surface plasmon polaritons. An experimental demonstration of the method is presented in which an electrically controlled interferometric configuration is used.