Code

function onLinkCreating(sender, args)
{
	var link = args.getLink();
	if (!link.targetConnection)
		return;

	var origin = link.getOrigin();
	var dest = link.targetConnection.node;
	if (origin == dest)
		args.setCancel(true);
}

function onLinkModifying(sender, args)
{
	var link = args.getLink();
	if (!link.targetConnection)
		return;

	var origin = args.adjustmentHandle == 0 ?
		link.targetConnection.node : link.getOrigin();
	var dest = args.adjustmentHandle == link.points.length - 1 ?
		link.targetConnection.node : link.getDestination();

	if (origin == dest)
		args.setCancel(true);
}