We know we have an "arachnid response" for things that are horrifying because of their fundamental alien-ness. We don't have a "giant water bug" response because everyone who's seen one is too busy looking for a heavy object and regretting not writing a will. And don't relax thinking that the giant water bugs restrict themselves to smaller prey, either. They've been seen eating things as large as snakes or even turtles, using their horror-monster face to inject flesh-dissolving horrorgoo. They've also attacked humans, and while they don't succeed in their plan to dissolve us like the Blob and slurp up the remains, the fact is that THAT IS THEIR PLAN. But instead of phase-changing us into supper, their uniquely awful poisons "only" cause hours of burning pain as bits of your body turn into flesh-smoothie.
The Japanese Ministry of the Environment lists it as endangered and is concerned about its survival. Come on, Japan, you murder whales as a snack and bulk-eat entire genii of fish, and THIS is the species you decide to keep around? Finish it off before the next nuclear accident gives it the gift of Godzillification!
How We Finish Them Off:
Pacific Rim. Jaegers. Giant hundred-meter-tall multi-weapon-enabled megabots to stamp them back into an organic slurry before they can infest our cities.
But if we have to fight them after they've been irradiated, build a Death Star.