Well, first things first you have to decide where to hide the body and how to dispose of it.
I would force them to drink bleach (a safer form) then ammonia and, before the chemicals had the time to react to each other, I would put them into an air tight small chamber and put them in wet cement. Once the cement hardened, drill a hole into the chamber then pour an acid into the chamber that would react to the mixture of ammonia and bleach and the chlorine that the mixture of the two would produce. This would strengthen the acid and completely dissolve the body.
Not the perfect plan as I don't know all the names of the compounds and an airtight chamber runs the risk of explosive chemicals, but since I don't plan on killing anyone, its not something I'm going to put thought into to perfect.
I won't get any more graphic than that but it is really amazing the things you come up with for hiding a body...