I was easily able to delete the Amazon-supplied free Kindle Dictionaries (and User Guides) from MYK. The problem is, every time you deregister and reregister, they come back...
Thus proving, by the way, that they are NOT gone forever.
And someone suggested in another thread the following bookmarklets for bulk deleting from MYK:
http://massdestroykindleitems.com/
http://pepa.info/check-and-delete/