If the value of the data justifies it, remove and physically destroying the hard drives is best.
A less extreme approach is to boot from a service CD and use the utility buried in diag that provides a three pass overwrite (two complementary patterns, followed by random) of your drives.
Physical destruction gives the best piece of mind, though, and isn't very costly if the machines are being decommissioned due to obsolesence.
Rod Knowlton
IBM Certified Advanced Technical Expert pSeries and AIX 5L
CompTIA Linux+
CompTIA Security+