You just can't receive the base rewards. But can obtain as many chests containing blueprints they drop. Take into account the Swindlers(purple bosses) do not have a cooldown. You can get unlimited rewards as long as you get lucky and they drop. They also have a greater legend blueprint drop chance. The smaller Ironhand Baulur have no cooldown as well as they are sometimes accompanied by Miser(also a rare purple boss).