We will delete a device from Central in case:
1. IAP got replaced due to hardware defect.
2. We do not want to manage it from Central.
All license (Device Management and Network Service) assigned to device should be unassigned, else if the device comes back up, it will start populating the monitoring information again.
We can delete the details of a device which is down, from the Monitoring page on Aruba Central.
Go to Monitoring & Reports App > Network Overview > APs > List of down APs > Select the AP and spot the trash icon at the end of it.