Exchange licenses in SBS 4.5

An Outlook 2000 client is getting a message that there are no more Exchange licenses available. I have removed old users but still the message appears. The License Manager in the Control Panel says I need to use the Server Monitor to change licenses but that seems to only allow for additional licenses.
On another NT 4 network (not SBS) I had the same problem and was able to go into License Manager and simply delete the obsolete users’ names and that freed up licenses.

Does anyone know howto free up obsolete user licenses in Exchange running on SBS 4.5?