when /opt/tivoli/tsm/db2/instance/db2ilist shows a DB2 instance, which cannot be removed by /opt/tivoli/tsm/db2/instance/db2idrop
/opt/tivoli/tsm/db2/instance/db2ilist
/opt/tivoli/tsm/db2/instance/db2idrop
enter as root:
# /opt/tivoli/tsm/db2/bin/db2greg -delinstrec instancename=<instance name>,service=DB2