Unregistering the database from catalog:
rman target sys/password@targetdb catalog rman/rman@catalogdb
RMAN> unregister database noprompt;
Unregistering the database that is not unique in catalog:
rman catalog rman/rman@catalogdb
RMAN> run
{
set dbid 3213213213; # DBID of target DB
unregister database targetdb noprompt;
}
Note: NOPROMPT - specifies that RMAN not ask you for confirmation before unregistering the database.