The only way to permanently destroy data, without physically destroying the hard disk, is by multiple overwriting of the data by generating and recording random characters across the entire surface of the drive, resulting in complete data destruction and resetting of file sizes to zero.
There are a number of overwriting standards which people refer to, however the current recognized UK standard is HMG IS 5 Enhanced, although there are many global equivalents, which can confuse the issue.
Computerhub uses White Canyon Wipe Drive 8.1 overwriting software, which is the only overwriting software in the world to be certified to NIAP Common Criteria EAL 4+ and is the only software approved by the likes of the Armed Services, Interpol, Homeland Security, IBM, Microsoft and Cisco.