When setting a package to remove while streaming a different package the hidden sessions will not terminate.
Symantec is aware of this issue and the appstream code does not allow the removal of cache during the period of when a layer is being installed. Once installation is complete it will allow for deletion as normal.
Use ITMS to create a policy to delete that layer from the cache.
Using Autoit and schtasks.exe you can create a pre/post event that creates a scheduled task to create a task that calls a compiled autoit script to run the appmgrcmd -r [guid] command outside the Vzservice and appstream service and drivers.