How do we prevent this from happening to our cars?
If you have euro spec manifolds (4 bolt flange) replace the pipe containing both the primary and secondary cats with a custom made or off the shelf solution with sports cats in the secondary cat position. Relocate lambda sensors, job done.
If you have the US spec manifolds (2 bolt flange) with the primary cats in the headers things become more complicated. You have a few options though:
1) Replace the US spec manifolds with euro spec ones and then do as you would for the euro spec above. Very expensive but the euro manifolds flow better so at least a small performance gain to go with your peace of mind.
2) Cut open the bottom of the US spec manifolds, gut the cats and weld them up again. Not ideal for the flow of gas in the exhaust but does deal with the problem effectively. Cut off the secondary cats and replace them with sports cats. There are no off the shelf options available for this (unlike the euro spec one) so you will either need to cut the original pipe or have an new one custom made. Then relocate the lambda sensors to the secondary cats.
3) The most common way of removing some of the worry is to simply replace the secondary cats and squish pipes with decat pipes, leaving the primary cats in the headers untouched. Not as full proof as the first two options but better than nothing.
All three options will free up the engine and may also add a little extra power, not to mention a whole lot of noise is you decide to mod the rear boxes or add a straight through X pipe.