Since you have no OS the easiest way is to pull the coin CMOS battery for 5 minutes.
We finally resorted to removing the battery for a minute to reset everything. We did manage to boot (via USB) into a Microsoft Disaster and Recovery Toolset that has a Win8 WinPE image on it. We ran UnProvisionEx.exe from there but it returned nothing. Just a plain C:\ prompt. Does UnProvisionEx.exe need .NET framework to connect to AMT?
Removing the CMOS battery resets all data in the MEBx back to their default values. If you've pulled the CMOS battery like Greg suggested, then the computer has already been unprovisioned.
Also, to answer your question about the UnprovisionEx.exe tool, yes you do need the .NET Framework for it to run.
Thanks Alan. That took care of it for us.