Adjusting the angular falloff and making sure to enable soft shadows will take care of your spot light issue.  If I want uneven lighting, like simulating the lighting underwater, I usually just create a transparency map image and either use it as a gel, or I create a flattened cube or square spline object and place it in front of a very wide angled spotlight at the desired angle.  And as said already, turn the ambient lighting under the scene in instances off (zero).  Sometimes using a few (2-3) spotlights with gels can create some interesting light overlap effects.