If you are trying to format a FileVault 2 drive, also known as “nuke and pave” on MacOS X 10.9 and you run into a volume that is protected with FileVault 2, you have two choices: turn off the encryption via FileVault and wait for that long process to complete, or use the diskutil command to delete the volume.
Boot up from your installer media (USB stick, etc) and go to the Terminal and do a:
diskutil coreStorage list
Then note the UUID of the CoreStorage logical volume group. Then do a:
diskutil coreStorage delete UUID
You will see a progress bar and your formerly encrypted volume will be gone forever. It may also be a good idea to delete all the partitions and reformat the volume just to be on the save side. You are doing a “nuke and pave” for a reason after all..