Alquin,
Back in 8.0 it was still safe and an accepted practice to delete computer systems from the NIS tables using SQL. From 8.1 forward, using SQL to delete inventory from NIS tables is not longer acceptable, because some data for computer systems is in UDM. We provided Discovery Server callbacks that safely delete inventory items from both NIS and UDM. I believe those were included in discovery server 8.12. We are working to provide a USM GUI mechanism to delete inventory items, but that capability is not yet available.
Jim Cooke, Product Manager, UIM Core Services