Hi MAF,
Data ONTAP implements storage virtualization by bundling disks into aggregates. On the aggregate you then create FlexVols, and potentially LUNs in the FlexVols, to store user data.
Assuming your aggregate layout is OK, to remove data on an aggregate just offline and destroy the FlexVols. If your aggregate layout is not OK, you have to offline, destroy, and recreate your aggregates. If you only have a single aggregate, and not enough spare disks to create a temporary small aggregate to hold the root vol while you destroy/recreate the current one, then you basically need to do a fresh install.
If your concern is about sensitive data on the physical disks, and you want to make sure that if someone took a physical disk from the NetApp system and connected it to a PC that no data could be read from it, then you need to destroy the disks and use sanitize for highest levels of security, or just reinstall with zero disks option if you don't need multiple overwrites.
Hope that helps.
Regards,
Chris Madden
Storage Architect, NetApp EMEA