Sure, I would love to help you with this. The steps that you have to follow to disable the HDMI output are:
1. Once you are in the BIOS setup (F2), go to Device Manager -> System Setup -> Uncore Configuration.
2. Go to the IGD Configuration section, and disable Integrated Graphics Device, this will disable the HDMI output.
3. After you have disabled it, just press F4 to save the changes and the system will restart with the HDMI disabled.
I hope you find this useful, and let us know if it works.
Have a nice day.