I think you could also use the remove subscriber properties and find this user to remove it. This tool is located in x:\commserver\utilitites\removesubscriberproperties.
Open the tool and it will look like your AD snap in. If you find the user that Unity states exists when it shouldn't and has a bunny icon next to it, you should be able to select it and delete it. It then will be totally cleared out of Unity.