You can wipe the disk using several methods (varied success has been reported with the dd commands). The full disk wipe (method 2) appears to produce better results but takes more time.
Method 1 (replace MBR method)
Zero out the MBR using a DD utility
1) Boot off a Linux Live CD (you will most likely not be able to modify the MBR while booted into the affected OS)
2) Run the dd command to wipe the drive (it's possibly you may have to install the dd utility using your Linux package management system for your distribution first)
dd if=/dev/zero of=/dev/sdX bs=446 count=1
Note: Replace "X" in the of field with the drive that is your boot drive.
Disk 0 (/dev/sda) is your primary boot drive
dd if=/dev/zero of=/dev/sda bs=446 count=1
3) If you are doing this on a system that hasn't had the operating system installed on it you should be fine to install the OS now if not, proceed to Step 4
4) Mount the / and /boot partitions
mount /dev/sda2 /mnt/sysimage (or whatever partition number is the root partition)
mount /dev/sda1 /mnt/sysimage/boot (or whatever the partition number is for the boot partition)
5) Mount /proc and /dev
mount -o bind /proc /mnt/sysimage/proc
mount -o bind /dev /mnt/sysimage/dev
mount -o bind /dev/pts /mnt/sysimage/dev/pts
mount -o bind /sys /mnt/sysimage/sys
6) Chroot into the environment
chroot /mnt/sysimage /bin/bash
7) Re-install grub boot loader
Method 2 (full disk wipe method)
Note: This method will require a restore of your data from backup and reinstalling the Linux operating system as it wipes the entire drive
2) Wipe the disk (usually just a quick wipe is sufficient).
3) Reinstall the operating system.
4) Encrypt the drive again.
Symantec Drive Encryption (formerly PGP Whole Disk Encryption)
Linux 2.6 kernel or newer