You may have disengaged the trigger from the sear, or it may have been like that when you got it. Easiest way is to carefully unscrew the two screws that hold the spring seat in place (at the back) and then lift it up slowly. That will raise the piston sufficient for it to clear the sear and un-cock the gun, although it will do so as it you are firing it. Then you'll need to drop the trigger block out of the gun (push through the pin that the fire selector stops against) to see why the trigger isn't releasing the sear. It releases by pushing up on the front of the sear, so pushing down on the rear where the piston is held in place. Could possibly be that the lug on the trigger is broken.
And leaving it cocked would not break the spring, just weaken it a bit.