If someone closes the stable door after the horse has bolted, they are so late in taking action to prevent something bad happening that the bad event has already happened. Here the word 'bolt' means to run away. • Improving security after a major theft is a bit like closing the stable door after the horse has bolted.
In the cartoon, the bad event is the fact that horsemeat has been found in a number of 'beef' products, the latest being Findus 'beef' lasagne. The man closing the stable door is an official from the FSA, or Food Standards Agency, the government department responsible for protecting public health in relation to food throughout the United Kingdom. The FSA has now ordered all British companies to test their processed beef products in the next week, but it has been criticized for not acting sooner.
On his blog, Adams says this about the cartoon:
I originally drew the FSA chap in farmer's outfit, thinking that he owned and (should have) controlled the horse. But then I realised you really need an authority figure to be clear about the negligence – hence the change of plan and him being clearly dressed as someone from the government.