@jaybeeq "Can some US citizen please explain this to the rest of us? "
This reference predates even the English language. Salting the earth was (in)famously done by Rome as a punishment to Carthage after victory in the Punic wars over 2,000 years ago. As Vydorscope stated, this would prevent the growing of crops.
The great orator Cicero was known for ending every speech, regardless of the topic, with the incitement, "Carthago delenda est!", "Carthage must be destroyed!"
My main problem with the comic strip is that it implies that PHB actually knows classical references and can use them properly.