Is there a way to make the lighting a square shape instead of a circular one?

Only by putting a plane with a white square in black border applied as a transparency map in front of it, and turning Cast shadows on for both light and square. The fall off within the sqrae would still be circular, but if you turned the Spread angle on the light well up that wouldn't be obvious (if you want noticeable square fall off there's no way to do it short of editing the shader for the light, which would change all of the lights of that type until you changed it back).