To my knowledge, this is not part of the features available on the QlikView server, nor through the API.
You can imagine that if such a feature were available, SaaS could easily be implemented on QlikView Servers, licenses could constanlty be reused, automatically diminishing the number of licenses needed for an enterprise's user community. From a customer's wallet point of vue, this would be great. But, from an editor's revenue point of vue, not as much!
Indeed this is not a standard feature of QlikView. However, it should be fairly easy to do if you know your way around building your own system scripts and utilities. There is a QV Power Tool for managing users and licenses. It can both grant and remove licenses.
See also this discussion Dynamic Doc Cal Deallocation for further suggestions.