Delete the mad scientist
You can delete him by right-clicking on the picture of the mad scientist and selecting delete from the popup menu.
Or, you could right-click on his item in the object tree and select delete from that popup menu.
Or you could drag his item from the object tree and drop it into the trash can shown near the top of Figure 1.
(He does an interesting little dance before he actually disappears from the world.)
At this point, your world should be back to the point that it looks very similar to Figure 1.