Note: the defib in question was the manual variety, which is why it delivered the shock. An AED (Automated External Defibrillator) shouldn't deliver a shock unless it needs to (it analyses the heart rhythm before deciding if a shock is necessary). Hence why they are positioned in public locations where anyone could have access to them (although you're still supposed to be trained to use one). I believe some AEDs do have manual overrides, but if someone was going to prank around with one of these they'd definitely be aware they were going to do some serious damage (i.e. murder).