See the code. Create a full screen costume and set it to the front with 100% ghost effect - it'll appear when the stop button is pressed.
I first saw this trick used by @Haulea in his Magnify project and thought it needed to be better known! UPDATE! Actually it turns out it is possible to keep running after someone presses the STOP sign - see http://scratch.mit.edu/projects/52503380/ for details!
