I have often thought about this... I would put the person in a Crab pot and toss them in the Cook Inlet in Alaska. Of course there would be no rope to retreive the pot. The thing about cook inlet is that there are huge tides that was in and out daily (30 foot swings are the norm). The water moves at roughly 7 knotts in or out about 20 hours a day so the pot would roll up and down the inlet and the sand fleas would get to them and the bones would be spread up and down the inlet with the tide. Oh ya, the water is very cold and full of silt so a diver can not find anything. Also the inlet is about 60 miles long... . that ought to work... .
A guy I know does the occasional illegal moose hunt (he eats them and needs the food). He disposes of the bones and such by burning them in a steel drum with lots of wood. Once they are ashes he dumps the barrel out and filters out all the teeth and small bones that did not burn and chucks them into an unknown location. Seems like that might work here too.
Am I freaking anyone out yet??
I also think the wood chipper hold promise, however if you watch the Discover channel you would know that a guy got caught doing it this way. Mostly it was because he did not clean up after himself and they where able to find the teeth in the lake he chipped his wife into. That and the fact that he rented the chipper and truck from a local rental place and they never cleaned it and called the cops.