After some futile searching, I was ready to boot up off of an OS X install disc (as one hint had suggested), but then figured I'd try one more thing -- I ejected the volume, and then, voila, I could format it as an unmounted volume. Easy!
I can now donate the PC to a youth artist group, knowing that the person who gave me the PC in the first place won't have all their data out there in the mean old world. I did a 'write 8 times format' in order to clear the drive. I point this out just in case anyone else decides to do this and accidentally puts a chunk of their personal data out into the world.

