Removing by your method will only remove APs monitoring stats from Central.
What the customer is asking is yanking IAP from someone's Central account, as he has bought it from him.
This would need proving to Aruba that customer ownership of device has changed. And that would need TAC intervention.
OR you can go the seller, and ask him to "unlicense" the AP from his Aruba Central account, and then the AP will no longer pull config down from Central.
Another workaround, is to block communication to "device.arubanetworks.com", which is responsible for directing AP to Central, so it picks its configuration from there.