You can make an input field where a user must type YES or type the name of the volume (perhaps after they selected the same from a drop down for example) as confirmation. You could also have the delete not actually delete but remove shares and then rename the volume to something like "deleted_20140313_volname". Then have another workflow that is run periodically that finds these "to-be" deleted volumes and actually deletes all volumes that are older than X days. A colleague of mine did this 2nd technique recently for a customer.
Thanks, Chris Madden Storage Architect, NetApp EMEA