If you look at the photo in this thread, the gear selector is the lowest control under the center air vent, to the right of the steering wheel. To switch to neutral you push the selector to the left (if you can see the 'N' there, click on picture for better view) for 3 seconds. Once in neutral, the power to the wheels will be cut.
Another option is to push the POWER button (you can see it to the left of the center air vent, near 2 o'clock mark on the steering wheel. Push it for 3 seconds, and the car will shut down regardless of what it was doing.
Prius doesn't have a steering wheel lock (and it doesn't need one, since it's virtually impossible to steal without a keyfob.)
The parking brake is a pedal to the left of where the clutch would be in a stick shift car. You can see a bit of it above the footrest. This brake pulls a cable that actuates rear brakes; it's no more efficient than in any car - I doubt it would have any effect on stopping a runaway car.
Prius has a kill button?