They can be selected by double clicking and deleted by pressing Delete. Or just selecting everything in the editor and pressing Delete. Tested with Chrome 53 and Internet Explorer 11.
I am confused as the description for Immutable Elements states:
"The Kendo UI Editor widget and its Immutable Elements feature enables you to predefine elements in the content area that cannot be edited by the end user."
I assumed that edited also meant deleted. Is this the intended behaviour?