Not sure how strict or proactive they follow up with their own clause:

Requirement to travel on your eTicket as bookedYou are required to travel on all sectors of your journey (by air, road, sea or otherwise) shown on your reservation and in sequence as shown on your eTicket. Where your itinerary contains connections (by air, road, sea or otherwise) within 24 hours, your check-in luggage will be checked through to the final destination shown on your eTicket. We do not accept you or your baggage for travel for only part of the itinerary. If you do not wish to travel on any of the sectors stated herein, please contact us to make changes to your ticket. You will be required to pay additional fees for the fare difference and the changes made, under the conditions stipulated in the ‘If you fail to fly’ paragraph of these conditions of contract. I haven't heard anyone getting smacked for not flying the last sector but I think if one is caught habitually doing it. They might start to crack down on it(?)

However, the clause does give them the power to charge the differences etc