Create a linux boot disk for powerpc Mac. You can download direct from ubuntu:
http://cdimage.ubuntu.com/lubuntu/releases/trusty/release/lubuntu-14.04.4-desktop-powerpc.iso
Use the linux command dd or shred to wipe your disk:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0ahUKEwjj7biAwcXMAhVD8WMKHQdyCzMQFgg8MAM&url=http%3A%2F%2Fubuntuforums.org%2Fshowthread.php%3Ft%3D778991&usg=AFQjCNHN_1metuUPsC0YKACD5pujouMnjw&sig2=sOqGHJy6W6nvRxDdK4zAmg