You should ALWAYS delete the subscriber from Unity FIRST and, also reassign any Call Handlers, etc that the Subscriber was listed as an owner of. Also, make sure that, if this Subscriber is tied to any Call Handlers, Distribution Lists, etc, that you have set up what is called a Substitute Object to handle any calls to these service handlers. Here is a doc:
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_unity/unity40/sag/sag405/dom/sag_0165.htm