my grandparents had a bar accross the inside of the catflap, meant you could open it about an inch, the have to let it swing back and catch it. Whilst pussy is busy doing that it cant hold an animal, so it has to let it go before it comes in the house.
It also had the effect of making the cat flap just a little too high to lift rabits through so even if it did ever work it out, it would have had a job getting the kill inside.
Doesnt work for ours though, theres 3ft walls topped with 3ft fences, with a further 3ft tot he top of the next wall, then a sloping roof, then a 3ft gap, then another 3 ft upto our cat flap! And the bugger can still get a full grown rabbit in!