With an opaque piece of cardboard (I usually mount a solid sheet of paper on one of my transparency frames). You may also turn off the projector completely, but beware, this can cause the projector bulb to burn out sooner. Overheating of projectors might cause melting of different parts of the device. This may lead to exposure of toxic smokes. If you are taking drinks or any beverage during the presentation, make sure that the projector is far off the glass.