Going slightly off-topic, coming from a legal background, I have never read any court cases of an airline trying to re-price a ticket for a skipped last leg and I think that if such a case ever reaches a courtroom it would be very hard for an airline to prove that any damages were done, actually in recent years EU legislators have discussed the issue of round trips and complex itineraries, there have been talks of regulating airline pricing strategies (for example forcing airlines to sell roundtrips at the same price of two one-way tickets).
The only valid claim an airline could make in court as far as I know is that by skipping a leg the customer may have caused a flight delay due to luggage having to be removed from the skipped leg.
There are some sanctions that airlines can employ without having to go to court:
1. Closing down frequent flyer accounts.
2. Potentially banning pax (won't be possible in the EU due to anti discrimination laws).
3. Obviously cancelling future legs of the itinerary.
4. Warning regular skippers that their actions are against T&C.