How can I erase hard disk if SuperDrive is broken (OS 10.6.8)?
You should try to create a bootable USB stick with some kind of recovery LiveCD (Ubuntu, BackTrack, ...)

I recommend using Ubuntu because it is the easiest to create.

There is a little tutorial on Ubuntu's website: http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx 
(If you Have trouble be sure to ask a question on AskUbuntu)

After you have a recovery usb wipe the disk using your favorite tool. I found a nice tutorial on 'How to geek', but it's a bit outdated.
(Tip: download wipe from the software center, not from synaptic)