The open door process at IBM is, at best, a joke. Don't waste your time with it. I went through it and I had a slam-dunk case against a superior (executive) that threatened me (voice and via email) career-wise if I didn't lie about something and then actually followed through with it. HR doesn't work for you, they work for Ginni and her minions.
Move on and find something else, I don't see any way you will overturn this decision.
get a corporate lawyer and push it.
I understand that Official Warning Letter is filed away with his permanent record from grade school.
Try request for independent HR who don't work for the executive team. The local HR is funded by the team who you are reporting to the team. They have a independent HR resources in Global , US, Europe and AP which has no ties with any of the team and not funded by any of the teams in IBM but they still pay by IBM. Request for that, it work for me before when someone above me is cheating within IBM on policy and procedure and trying to make a scene to show his intimidation over me. Eventually he got a official warning letter.
But of course if it was one of the pee ons that commited the crime you would be held accountable to full extent of the law ...fired!
pretty much