Create a custom script (in Visual Basic if you want) that uses QlikView Power Tools to first extract all licenses and the date of their last use, and then uses another Power Tool to delete those license assignements that haven't been used in three months. Make the delay configurable, so that you can shorten it when needed.
Power Tools:Power Tools 1.2 for QlikView
This question has been asked before. I think you can find further help if you do a community search.