Hello,
I had a similarly difficult touchpad to disable, and I got around the problem this way. If possible, plug an external mouse into a USB port, then open the Touchpad control properties tab and look for a setting worded something like this: Disable Touchpad when an external mouse is detected.
Then go back to Device Manager and try to disable the Touchpad. Since an external mouse is already plugged in and I therefore have a registered pointing device, Windows would let me disable the Touchpad without reinstalling it on the next reboot.
Let me know if this fixes the issue for you.
Regards,
BearPup