How to easily and completely disable/enable WLAN (so it cannot be turned on again while travelling)?
Discussions of willpower aside, my research suggests that the file you need to copy to the USB stick and later restore is /System/Library/Extensions/IO80211Family.kext. This will require a reboot to take effect.

Before removing any system files, be sure you have a working bootable backup of your system and time on hand to restore things should anything go wrong. I have not tested this solution on my system and am not willing to do so.