Since the garage will be demolished it won't matter what you do to it. To keep it cheap I would take and nail boards across the doors and make it look off limits. Then a collection of flashing red, blue or green lights and a boom box playing weird noises or screams set up inside. Maybe whispering voices for the sound. I picture an old garage with the walls made up of vertical boards where you could see light coming thru the cracks. If that's not the case you could cut holes or slits in the walls to let light thru. Make up a story of a gruesome murder that happened there and have that as the excuse for tearing it down. Tell the story and only agree to show people reluctantly and have the lights and sound ready or looped. You could take an ax and chop up the door before you board it up. Maybe cut a hole in the back wall for access. If there are any windows, break them and board them up. Think gruesome murder crime scene and then do what you have to do...