This image that installs when you bring a blank computer up on the network sounds like a PXE boot. If that's the case, then the imaging software that boots is likely WDS/MDT or SCCM.
In the BIOS of your computer, set the network as your first boot device (or you can option+boot, but the key you use is different for every PC manufacturer).
Once you've booted to the network, there should some kind of dialog that pops up to begin the imaging process -- either a confirmation or a screen to enter your network password. At that point, you can press either F8 or Shift+F10 to get a command prompt window.
From the command prompt, enter the following commands:
diskpart
select disk 0
clean all
That will zero out the whole drive. Note that it could take an hour or more depending on the size and speed of the hard drive, and you will get no progress bar of any kind. Just a notice when the process completes.