Flea powder would have been great idea then.
Not killing off the cats that kept the rats carrying the fleas in check wouldn’t have hurt either.
Remember, when you see a large number of cats in an area, that’s a symptom of a larger problem. It means that there is a VAST food source for the cats, and usually that’s some sort of vermin. Eliminate the cats, and whatever they’re keeping in check will explode - usually with very, very bad results.