Here's what you do. Knock them unconscious with either a ruffy or something that doesn't make a mess. You then get some Potasium Chloride(Spellcheck?) and send it down their throats. You can get some just at a local jewelry store, just say you are trying to renew some old jewelry. A milligram of that should do the trick. After they swallow it, they should be dead within a minute.
You then need to line your bathtub with the clear sheet that you line the floors when you are painting a room. Make sure you get every inch of that bathtub. You can then either put their head towards the drain, or hang them upside down somehow. Slit their throats and wait about a day for all of the blood to drain out. Wash said body down with the shower head. Go down to your home depot and grab some toilet acid. The kind of stuff you put in toilets to completely unclog it. It usually is in a bag and has a lot of warnings. While you're there, you should pick up some Waders, a pair of rubber latex gloves, and a metal bucket, maybe three feet in diameter, and an apron. If you can't get the bucket, try a plastic pool at your Toys R' Us, but I don't know if that will work.
Grab the drained out body, and put it in the bucket. Pour all your acid on it, and wait about a half hour. Make sure you are wearing all your equipment (Gloves, Apron, Glasses, Waders). Step on the body, and the acid should have corroded it enough were it will break easy. Mash is up really good where it's like a salsa. You then have a couple of options. I would personally, get some cement mix, and use the mashed up body as the water needed for the cement. Toss the cement block in the ocean(If available) or just bury it. Dealer's choice.