If you go to setup --> Assets --> Discovery Connections --> click on your casper connection --> click the "edit" pencil --> the option when assets are removed, you can have it delete, no action (do nothing) or set the status to a custom choice.
We have ours set to out of service. then if we send the unit to the electronics recycler we manually change it to recycled.
We felt this was better because then if the asset is assigned to any tickets all that data stays put vs just deleting it.