If the person is a drug user, by a temporary cellphone and call him up to ask if he wants to hang out. Leave him a little something and poison his coke or whatever, then he'll die.
Otherwise I think you all are overcomplicating this. The more work you put into the more can be ued against on loose ends. As long as you haven't made vast declarations of distrust or enmity. Just go and randomly shoot the guy, and let it be overwith. Shoot him and burn down his house etcetera.
Even better, cut the gas main and fill his house with carbon monoxide while he's sleeping.
People WILL notice this person is missing, they WILL start an ivnestigation of possible murder even if you vaporize the body.
I think quick and simple is the best bet.