What can I do about an old steam baseboard heating valve that wont turn?
As a temporary solution you could cover the radiator with blankets or clothing to limit the amount of heat dissipation into the room. Don't worry, hot water/steam heating systems do not get hot enough to ignite combustible materials. 

If you cannot get the valve repaired or replaced, then you may want to pick up some foam insulation boards and duct tape from a builder's supply store. You can cut the foam with a sharp razor knife to form a 3-sided box that would fit over the radiator. Then just duct tape the edges of the foam box and it should set in place against the wall. Adjust the distance the box is from the wall for regulating the heat in the room.