I am like every one very angry when I hear stories like this, it has been a problem all over for awhile and nothing seems to be able to stop it, it is a challenge that has also been faced by hikers and mountain cycles.
The reality is, that it is the way it is and wishing it away will not help. When I was a kid we never locked the doors to our house or cars, things changed, it is less convenient but we had to change our habits and lock stuff.
The same I guess has to happen at the crags, I have some tips that I believe will help:
1. Plan you trips as far in advance as possible and try and get at least 5 or more people together.
2. Don't all be on the rope (climbing Belaying) that is when you are most vulnerable.
3. Be very conscious of you surrounding.
4. If you see suspicious people or are note comfortable leave, don,t climb.
5. Climbing on rock in this country is very cheap, you really only pay for the trip, so if there are 6 people club in R 15 bucks each x R 6 = R 90 that can be used for a car guards or two. Then what you do is let the farmer know when you are coming and ask if he can arrange some one he knows and trust for you. This could be a big deterrent as a person from the area is more likely to recognize the people involved.
Long story short, take the temptation away.
The important thing is that these guys are given as few opportunities as possible to get these attacks right, with every one they get right they get more confident and will become more brazen in their approach.
I believe that some one should try and get some kind of electronic calendar going, then get every one in the industry to support the initiative, the websites, publications, MCSA as well as gyms and other places where climbers mix. Does not need a lot of info, date, place, time number of people climbing.
Catching these guys can be done but would take a lot of time and effort, you could set up a trap and bait it with a very couple in a nice car. You then chase the guys down and do a citizens arrest. Remember there will be the court appearances and all that inconvenience as well.