You will have to loosen spokes in the area of the flat spot and tighten the spokes either side. Then squeeze in the walls of the rim with vise-grips. Careful of the power of vise-grips as they can destroy a rim in milliseconds. If only one side of the rim needs to be squeezed in put a flat piece of metal (like a cone wrench) between the other side and the vise-grips to spread the force over a wide area. When the side walls are straight, go back to the spokes to get the rim true. You probably wont be able to make the wheel perfect with this technique but you can make it a lot better than it is now.