Make content non-selectable

I'm trying to make it so a certain element in the editor is non-selectable (either directly or with highlight).

I tried elemet.unselectable(); but that didn't seem to work.

I got as far as this to prevent you from directly clicking on it (if you do, it moves the selection elsewhere):

var sel = new CKEDITOR.dom.selection( ev.editor.document );
elem.on( 'click', function() {
	sel.selectElement( elem.getParent() );
	});

But you can still highlight it by selecting a different element and then highlighting. Is there any way to avoid this?