It is impossible to eliminate rising heat from an air duct. It is possible, however, to reduce the amount of heat that gets through the floor. There are many joints in a standard duct system. These joints are natural air loss areas. If you can gain access to the duct under the floor, sealing these joints would improve the situation. In addition, install some insulation over the duct and underneath the floor. This will hold the heat in and what heat does rise will be more evenly distributed and not localized in one section. If adding the insulation does not help, you will have to pull up the cushion flooring and glue it back down.