Ans:

The methods of the Event class can be used in event listener functions to affect the behavior of the event object. Some events have an associated default behavior. For example, the doubleClick event has an associated default behavior that highlights the word under the mouse pointer at the time of the event. Your event listener can cancel this behavior by calling the preventDefault() method. You can also make the current event listener the last one to process an event by calling the stopPropagation() or stopImmediatePropagation() method.

for example:

package {

     import flash.display.Sprite;
     import flash.text.TextField;
     import flash.text.TextFieldType;
     import flash.events.TextEvent;

     [SWF(width=550, height=400)]

     public class Main extends Sprite {

          public function Main() {
               var tf:TextField = new TextField();
               addChild(tf);

               tf.width = stage.stageWidth;
               tf.height = stage.stageWidth;
               tf.type = TextFieldType.INPUT;
               tf.wordWrap = true;

               tf.addEventListener(TextEvent.TEXT_INPUT, onTextFieldTextInput);
          }

          private function onTextFieldTextInput(event:TextEvent):void {
               var tf:TextField = event.target as TextField;
               if (tf.text.indexOf(event.text) > -1) {
                    event.preventDefault();
               }
          }

     }

}