You can delete objects by hovering over an object and clicking on the three dots underneath it. You'll see the "delete" button in a drop-down menu.
You can also delete all of the objects in the room at once by going to the "Room Details" tab in the settings icon at the top of the room. Scroll down to Manage Content and click the red "DELETE ALL" button. This will delete every element in the room, essentially resetting the room. The only thing that will stay the same is the background.