This does not appear to work this way on AirDroid 3.x. I did not sign-out of AirDroid on my old Nexus 4 before wiping the phone. My online account would still show the phone in the device listing, but not give me an option to delete.
As a work-around, I re-initialized the phone, connected it to Wi-Fi, downloaded the AirDroid app, signed-in to AirDroid with the device, then signed-out of AirDroid on the device. The Nexus 4 then disappeared from the device listing in my AirDroid account.
Luckily, I still had the phone in possession to do all of this. I then reset the old phone to factory settings to wipe it clean again.
This must be a bug. I should be able to remove any device from online admin in AirDroid 3 Premium. The Nexus 4 was running Android 5.x which which does not appear to be fully supported by AirDroid yet. That may have been the source of this problem?
