Troubles to cancel command

Thanks for reply.
I didn't test it yet, but i think 

evt.cancel();

cancels the "beforeCommandExec" event and not, from your example code, the "paste" event.

-- edit
 

editor.on( 'beforeCommandExec', function ( evt )
{
	var mode = evt.data.commandData;
	// Do NOT overwrite if HTML format is explicitly requested.
	if ( evt.data.name == 'paste' && mode != 'html' )
	{
		editor.execCommand( 'pastetext' );
                // evt.name = "beforeCommandExec" and not "paste"
		evt.cancel(); // cancel the "beforeCommandExec"
	}
}, null, null, 0 );

Its should cancel the "beforeCommandExec" event not the "paste" event.
The function returns "undefined" and thats will changed to "false" (CKEDITOR.event.fire)
false !== true -> true, so exec() ist called.

Your example looks similar to my code ;)
My purpose is to cancel the enter/shiftEnter command on certains elements.
My full code:

(function(){
  // Check if selection is a figure or figurecaption
  var isFigure = function(e){
    var selection = e.getSelection().getStartElement();
    var name = selection ? selection.getName() : "";
    return (name == "figure" || name == "figcaption");
  }

  CKEDITOR.plugins.registered.enterkey.beforeInit = function(editor){
   editor.on('beforeCommandExec', function(
     if((evt.data.name == 'enter' || evt.data.name == 'shiftEnter') && isFigure(e)){
       // evt.cancel(); not working here
       return false;
     }
     return true;
   });
 }
})();

But let see https://github.com/ckeditor/ckeditor-dev/blob/master/core/editor.js#L821 again

if (this.fire( 'beforeCommandExec', eventData ) !== true ) {
  eventData.returnValue = command.exec( eventData.commandData );

  // Fire the 'afterCommandExec' immediately if command is synchronous.
  if ( !command.async && this.fire( 'afterCommandExec', eventData ) !== true )
      return eventData.returnValue;
}

If beforeCommandExec return false, this.fire('beforeCommandExec', eventData) returns also false 
If  beforeCommandExec return true, this.fire('beforeCommandExec', eventData) returns an [Object obj]

 if ( false  !== true ) {
 // No canceling     
 returnValue = command.exec( eventData.commandData );
}

if( someObject !== true){
 // also no canceling     
 returnValue = command.exec( eventData.commandData );
}

You can see the condition is totally useless, because its everytime "true"

Bug was reported and confirmed: 

https://dev.ckeditor.com/ticket/11413
Will probably fixed in CKEditor 4.3.3

My temporary fix for this issue:

(function(){
  /**
   * Enterkey: No enter/shift event on figure or figurecaption
   */
  CKEDITOR.plugins.registered.enterkey.beforeInit = function(editor)
  {
    // Check if selection is a figure or figurecaption
    var isFigure = function(e)
    {
      var selection = e.getSelection().getStartElement();
      var name = selection ? selection.getName() : "";
      return (name == "figure" || name == "figcaption");
    }
    editor.on('key', function(evt)
    {
      var keyStroke = evt.data.keyCode;
      // enter or shift + enter
      if ((keyStroke === 13 || keyStroke === CKEDITOR.SHIFT + 13) && isFigure(editor))
      {
        evt.cancel();
      }
    });
  }
});

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