After deleting the discovery_agent probe from the unwanted robots it will need to be removed from the database with the following commands:
1. Find the discovery_agent "da_id" you wish to remove by running the following
select * from CM_DISCOVERY_AGENT
2. To remove, use the following query to execute a stored procedure, entering the da_id at the end of the query.
exec P_CM_REMOVE_DISCOVERY_AGENT @da_id = <da_id from previous query>
Note: this will remove the scopes associated with that discovery_agent.
Simply refresh USM and the unwanted discovery_agent should now be gone.