As Steve pointed out, it all depends on YOUR requirements in regards to wiping data. For example, does your company have to meet Department of Defense (DoD) requirements for wiping data? Perhaps to pass an audit for Sarbanes-Oxley, PCI or the handling of SSNs?
If you don't have to meet DoD standard, then destroying the vol/aggregate and zeroing the disks will work just fine. It will delete the entire file layout and make a single-pass zero of the disk.
If you do have to meet DoD requirements, zeroing-out the disk drives will not meet that requirement and isn't likely to pass an audit. DoD requirements stipulate writing multiple data patterns, in multiple passes (six I believe), over the disk drive to ensure that no block written to the drive could ever be recovered. This is why you would need disk sanitization license. Host-based DoD wipe utilities won't necessarily work with NetApp because of the way we virtualize blocks.
Last time I checked, disk sanitization licenses were free. Talk with your NetApp sales representative and they can assist you in obtaining a license key.