Yes. There should be two control systems there now. If not, they can be installed. On the ducts that extend out from the furnace there are handles that control a small damper inside the duct. You can use those to control the amount of hot air that flows to each floor register. The other area where you can control the amount of heat is in the bathroom itself. The vents, either on the floor or on the wall, should also have a duct control that will allow you to reduce the amount of hot air flow. If the duct is rising straight up and the bathroom is directly above, the bathroom is probably receiving too much air that should be spread throughout the house. You could disconnect the current arrangement and reconnect the bathroom duct to a position 90 cm (3 ft) or so away on the side of the main duct. This too would reduce the air flow to the bathroom.