write zero to the partitions from a LIVE DVD or RESCUE DVD and that will erase everything on those partitions. then delete and re-create, or else do whatever you want to do.
I have Qubes running under Qubes when I want to trial things.
So just use DD from Linux, whatever version you want to use.
# dd if=/dev/zero of=/dev/sdXY bs=4096
Then fdisk to delete and create partitions, or just re-install Qubes.