How can I wipe an encrypted volume when the password is forgotten?
To delete encrypted volume disk (not your main boot disk)


Open Terminal.app
Unmount disk using

diskutil unmountDisk force /Volumes/disk2 (substitute /Volumes/disk2 with your disk)
Find disk UUID

diskutil coreStorage list
Delete disk/volume

diskutil coreStorage delete [UUID] (substitute [UUID] with your disk's UUID obtained from the previous command)