No way to disable touchscreen was found so far. (I bet RCE wizard igorsk could figure it in a couple of hours if not faster) It's not even clear, if it is on or off when reader is connected via USB. Trying ot figure it with camcorder with "night vision" also rendered no results.
Anyway, main idea behind this was to consume less power (touchscreen on x50 should consume quite a bit). "Disabling" touchscreen by software means, i.e. simply ignoring all input is definitely possible and will make it into PRS+, but don't expect it in the next release.