This is possible to do, although it needs to be done correctly. The supported method of course is the way TomMC described. If you are careful, you can do it this way - as long as you don't want to retain ANY of the files
***THIS WILL WIPE THE DRIVE CLEAN AND REMOVE ALL DATA SO BACKUP BEFORE DOING***
Both ways take several hours to do but may be faster than the decryption process depending on hardware and PGP version that is installed.
You have two options basically:
1) Take the drive out of the mac and put it in a PC with windows and use diskpart to clean the disk. Here is a KB on that process:
http://www.symantec.com/docs/HOWTO55966
2) Use DD utility to wipe the drive:
You can boot into single user mode (it's tricky while bootguard is on teh drive and encrypted) you have to press Command-S IMMEDIATELY after putting in your passphrase at bootguard. Another option is you can add a pgpwde --bypass command to bypass bootguard once. Thsi can be done via terminal utility with:
pgpwde --add-bypass --disk 0 (or disk number 1 if secondary drive) -p password or --wdrt (for wdrt)
once you do the bypass, the next reboot will bypass bootguard and you can hold down the Command+S keys and this will boot single user mode.
Another option is to just use the Mac OS Install disc and boot up to the install then go to the Menu at the top and select terminal
Wipe the drive using DD:
dd -bs=4096 if=/dev/zero of=/Volumes/DISKVOLUME HERE
You can find the volume information with:
diskutil list
Once you use DD to wipe the whole drive with zeros this will wipe out the PGPMBR and the encrypted sectors and allow you to reinstall again