If the boards are glued together, the easiest way to remove them is to heat the joints with a heater gun. This technique will soften the glue enough, making it easier to separate them. If this doesn’t work either, you may have to pry the boards apart, but this will stop you from reusing the boards again if you decide that you wanted them installed in another room.
Warning! Please do avoid trying to pry the planks apart with a pry bar or screwdriver, as both of these tools will leave noticeable marks and damage on the edges of the boards.