How can we completely remove a robot from UIM so that it no longer comes back to discovery? 

What is the correct procedure to decommission a system from UIM? 

UIM 20.x

Step by step instructions for removing robots or hubs from the UIM Inventory that is visible in the Operator Console

It must be noted that Infrastructure Manager (IM) and OC Inventory have different methods for querying a robot. IM handles the hub-to-robot communication (e.g. hubs pinging robots to verify if they are active). OC inventory handles the discovery correlation that it gathers directly from the database. For this reason, the robots need to be removed from both IM and OC as the delete from inventory in OC does not have visibility into the IM client. 

If the robot is only removed from the OC Inventory, then the hub will still think that the robot exists in the IM client.  The reason for this is the robot will show in a down state in IM because it has not yet been removed. 

When the hub does the ping requests to this robot, it will send a robot inactive alert.  After removing the robot from IM as well, this, will correct the issue.

NOTE: For a hub see the HUB NOTES sections included below:

Remove a robot in UIM 20.x.x.

HUB NOTE:
For hub first move all reporting robots to another hub.

1. Make sure you do not monitor these inactive robots from remote monitoring probes like rsp, net_connect, ICMP, vmware etc.

2. Stop the Nimbus Robot Watcher service for the robot that needs to be removed OR uninstall the robot from the system directly. 

 
HUB NOTE:
delete any named services setup or tunnels setups connecting the decommissioned hub

3. Remove the robot from IM. This can be performed one of two ways: 

* Remove the robot from IM in the hub robots tab. 

--OR-- 

* If there are many robots that need to be removed, the robot.sds file can be renamed from the hub folder structure in nimsoft/hub, for the hub that the robots report to. The hub will then need to be restarted to have the robot.sds file recreated. 

HUB NOTE:
Remove the hub.sds file from any hub that had a named services or tunnel connection to the decommissioned hub.
 

4. Remove the robot from the Inventory section of OC to delete the alarms, QoS, and all correlation related sections in the database for the robot. 

Follow the steps From the following Tech Doc:

Remove Devices in Operator Console (broadcom.com)

UIM - Automate the process of robot removal from Windows (broadcom.com)