[flexcoders] Drag and Drop question

[zootpeet]

I am trying to implement drag and drop onto a tree. In the dropDrop
handler, I want to be able to deny the drop from happening, based on
which element of the tree is being dropped on. 

  // r is the visible index in the tree
  var dropTarget:Tree = Tree(event.currentTarget);
  var r:int = dropTarget.calculateDropIndex(event);
  dstXmlTree.selectedIndex = r;
  var node:XML = dstXmlTree.selectedItem as XML;

  if (node.toXMLSting == "whatever") {
     // Allow drop  -- HOW?
  } else {
     // Do NOT allow drop  -- HOW?
  }

How do I deny the drop?

thanks!