1. If the license expires:
When license expires—>Device get unlicensed OR you can say subscription is unassigned, device won’t connect to central (local UI is fully functional now for complete set of operations on IAP).
Central users still can login into the the Central portal and can always add new licenses and begin assigning them to devices.
The device for which license is expired will show down in Central.
In case of customer already having a different valid license, then, devices get automatically assigned to that license and continue the operations.
2. If license are still valid and you want to remove the devices from Central. Just unassign all the license of the device, device management & service, both, in Central. This will remove the device from Central and local UI will come up with complete functionality.
Central will show device as down and you can remove the down entry, from monitoring pane by following:
https://community.arubanetworks.com/t5/Controller-less-WLANs/Delete-details-of-down-device-from-Aruba-Central/ta-p/291887