Did you look through the FAQ? Item #7 might help:
http://www.faronics.com/faq/default.asp#7
Will the computer boot into Target Disk mode (holding the T key while booting)?
If so, you can connect it to another Mac via Firewire and use Disk Utility on the other Mac to erase this computer's hard drive.
If Target Disk Mode doesn't work, try booting into Single-user mode by holding command+S on startup. I, unfortunately, don't know the command(s) you would use to get the HD erased, but if the computer will boot into single user mode, there's some hope.
Last thing to do is remove the hard drive, place it in an enclosure, and use another Mac to erase it. If your computer is under warranty, you'll want to be careful removing and reinstalling the drive.
If none of the options above work (or, in the case of the last one, you don't want to take the risk of damaging things), you'll have to bring the computer to an Apple store or authorized service provider and have them take a look.
~Lyssa