Now that I think about it... there is a 100% effective way for airlines to prevent passengers from exploiting hidden city pricing. Just charge upfront for the cost of the most expensive leg of a multi-leg trip and refund the difference when the boarding pass is scanned on the last leg. In the event that the complete routing is more expensive then any individual leg, just charge the regular price and leave it at that.

Getting this system up and running would be trivial for any competent IT department. Of course the PR department would have a tougher task on their hands selling this to the public, but hey, the airlines are blameless in continuing to perpetrate their insane pricing scheme and anyone who finds a way to use it to their own advantage is just a dirty thief, so I'm sure the public will be won over in the end.