Devices are not added in an ad-hoc manner. They are discovered by the Availability Manager, and the Service Assurance Manager server subscribes to the topology and notifications that are managed by the Availability Manager . Discovery can be seeded, ad-hoc, or automatic using filters.
Devices first need to be removed from IP domain and then they will automatically be removed from SAM after topology synchronization completes. If you have deployed the OI domain or any other analysis domain that subscribes to the IP domain, then those domains need to synchronize their topology with IP Manager and Service Assurance Manager to completely remove the device.