def unsubscribe(requestor, service, nodeIdentifier, subscriber):
    """
        Called when a subscribe request has been received.

        @param requestor: The entity the request originated from.
        @type requestor: L{JID<twisted.words.protocols.jabber.jid.JID>}
        @param service: The entity the request was addressed to.
        @type service: L{JID<twisted.words.protocols.jabber.jid.JID>}
        @param nodeIdentifier: The identifier of the node to unsubscribe from.
        @type nodeIdentifier: C{unicode}
        @param subscriber: The entity to be unsubscribed.
        @type subscriber: L{JID<twisted.words.protocols.jabber.jid.JID>}
        @return: A deferred that fires with C{None} when unsubscription has
                 succeeded.
        @rtype: L{Deferred<twisted.internet.defer.Deferred>}
        """
