As well as the suggestions for using dd, dban and shred, there are a couple of other options that may be possible, depending on your system.
If you have disks that support full disk encryption (also known as self-encrypting disks), you can request that the disk change the encryption key it uses. This will result in the disk immediately becoming unusable, as all the encrypted data on it is now unaccessable. Modern SSDs support this, as do many modern SAS disks (eg, Seagate Constellation ES SAS / Seagate Savvio / Seagate Cheetah). All disks that support FDE/SED are always encrypting the disks, regardless of whether you have some kind of encryption infrastructure to manage keys set up over the top - so you can always securely erase these disks by requesting a key change. (I don't have any handy docs on how to do this, and I can't remember where my research led me to last time I looked at it. )
Another option is to use the ATA Secure Erase command, which does a low-level zero write over the entire disk. This NIST Guidelines for Media Sanitization document states that the ATA Secure Erase command is good for security requirements up to purge level, which probably means it's good enough for you.
Neither method may be available to you, and I think a single dd or shred run is probably the simplest thing for you to implement, however there are other options :)