It is easy to extend the 'unselect' method.
<script>
$.extend($.fn.tree.methods,{
	unselect:function(jq,target){
		return jq.each(function(){
			var opts = $(this).tree('options');
			$(target).removeClass('tree-node-selected');
			if (opts.onUnselect){
				opts.onUnselect.call(this, $(this).tree('getNode',target));
			}
		});
	}
});
</script>
The code below shows how to unselect the selected node.
var t = $('#tt');  // the tree object
var node = t.tree('getSelected');
t.tree('unselect', node.target);