Hi,
One way from the client side would be listening to UserEvent.USER_REMOVE event from UserManager. This gets fired when the user item gets retracted from on server.
We mainly use this for any client side bookkeeping of userlists.
However, using server to server API your server can listen to the UserManager Collection once you have called registerhook, and subscribed to UserManager Collection. There is a php example in sampleApps for your server registering hook and listening to a collection node like UserManager.
Thanks
Hironmay Basu