Deleting a hard drive protected by password with Mac OS X Yosemite
Solution 1:
If a partition was encrypted with FileVault, and you have forgotten your password, you need to boot into the Recovery Partition to format the partition / drive. (Hold down Command+R during startup to get there.) From there, you can use Disk Utility to erase the encrypted partition.
Solution 2:
You can simply wipe the disks. Wipefs or even parted could be used, or you could just write zeroes to the drive using dd.
The osx tools have some protection against deleting partitions in case of a corestorage being there.