Hi you cat play around with the .pgo files but you can't just go deleteing them you need to replace them with copies that didn't have the cals assigned. this is a bit messy though, have you looked at the Power Tools, in their is a command line tool for releasing cals that you could set up as a scheduled task?
I'd use the QlikView PowerTools (qv-user.manager) in a Windows batch file and using either the Windows Scheduler or the QMC external tasks to run it on a daily basis to delete the assignments with more than 24 hours without use. I don't have the exact code, though, and I'm not sure it's worth the time to develop using any external tool to check the results of the query, then run the PowerTool again to actually delete them.
Bear in mind that, in all cases, this is not officially supported by QlikView, and you might need to check the licensing plan you already have with your Partner or Sales Manager just in case there are more suitable configurations for you or your customer.
Hope that helps.