Answer
There is no direct way to terminate a Plesk license in Control Panel. The license is created and installed into Plesk upon provisioning and is terminated upon unprovisioning.
However, when PVPS resources are removed from a subscription, there is no Plesk uninstallation from the VPS, so it is possible to create a downgrade order and remove all PVPS resources. In this case Plesk inside the VPS will remain without any active license and will not be manageable in CCP.