I had the same problem with my wife's XPS 17. I needed to restore basic functionality so disabling the touch screen was acceptable. However, unlike Vikin (but like you) I didn't have the "Use your finger as input" option in the control panel.
I tried to disable the N-Trig device driver in the Device Manager but the driver did not seem to have be correctly installed (after upgrading from win7)
I went to the N-Trig driver download site (http://www.n-trig.com/Content.aspx?Page=Downloads_Drivers) and downloaded the new drivers for windows 8 (I have the feeling they were not there last week).
I then when to the "Device Manager" options on the "Hardware and Sound" page of the Control Panel.
In the Device Manager display I saw a N-trig DuoSense Digitizers node with several sub-node underneath it (before dev driver install there was only one).
I right mouse buttoned on the item at the top, with a name ending in "Root Interface" and selected "Disable".
This successfully disabled the touch screen.
Shame to lose the touch screen but my wife did not use it and I was in danger of my life as it was.