From power off and sd ejected, hold down power and volume down to get into Android recovery
Connect the device to your computer via USB using the "DC" port on the device.
Switch ADB into root mode using adb root - You may get a timeout error here, continue on anyway.
If you would like a backup of your Android partition, run adb pull /dev/block/mmcblk0 android.img
Get an ADB shell with adb shell
Wipe the Android partition: dd if=/dev/zero of=/dev/block/mmcblk0 bs=4M This will take a few minutes and when it is done you will get an "out of space" message.