Prototype Event stopObserving() Method
This method unregisters an event handler.
This function is called with exactly the same argument semantics as observe. It unregisters an event handler, so the handler is not called anymore for this element+event pair.
Event.stopObserving(element, eventName, handler [, useCapture = false])
Here are the explanations about the passed parameters:
element: The DOM element you want to observe; as always in Prototype, this can be either an actual DOM reference, or the ID string for the element.
evenetName: The standardized event name, as per the DOM level supported by your browser. This includes click, mousedown, mouseup, mouseover, mousemove and mouseout.
handler: This is the event handler function. This can be an anonymous function you create on-the-fly.
useCapture: Optionally, you can request capturing instead of bubbling. The details are in the DOM Level 2 Events
This example shows how it reacts only once clicked and after that program stops observing.
To understand it in better way you can Try it yourself.