tie them to the bottom of a boat and then drive around near the rocks on the surface. just before they're about to get one of their organs hurt untie them, and sprinkle salt into their open body. then seal them into a coffin and leave it out to float in a swamp. feed them way too much honey and milk everyday. they will have to release feces, which will clog their own coffin. make a small hole for breathing in the coffin. flies will slowly enter and eat the feces, then slowly devour the person until nothing is left but rotten flesh. even in death they don't get respite. burn the coffin and naught will be left but ashes. throw these ashes in a box, and bury the box deep inside the earth. a year later dig it up, and empty the ashes into a box of uranium waste. send the uranium waste and the ashes into the sun.
problem solved.