There are two parts to adding a device in Central.
1. Add it in device inventory. This step is just a proof of device ownership. At this time device is not licensed. You can add a device in device inventory by serial & mac address, Activate, cloud key etc. Or Aruba will auto populate this for you based on devices sold to the customer name in purchase orders.
2. Assign device a license / subscription. After adding a switch you can assign it a license or subscription and device starts getting managed by Central, and comes up in monitoring pane.
Now, if you want to remove the device from monitoring pane, unassign device the license, and remove it from monitoring, based on my previous message.
And finally if you want to remove it from the inventory, then you DO NOT have the trash can icon in inventory! Then how do you remove it?
At this point as it is not assigned a license by you, someone else can add it in their Central account.
This act of someone else adding it in their Central account inventory, will remove it from your Central account inventory.