Put them in a room and remove any means of escape (including being able to hit me over the head with something and run), but otherwise treat them really well. Give them TV to watch, bring them decent food, give them a nice cushy mattress to sleep on.
Eventually the frills of being treated well will start to wear thin, and the reality that they can't leave the room will become ever more apparent. They might lash out a bit at first, but eventually start to break down. They will slowly fall into a deep depression that the accommodations won't help. Day by day, their humanity will be removed. Their old life will become a distant memory. They will cease to be anything but my prisoner.
In time, they may develop Stockholm Syndrome and begin to love their surroundings. They may even become accustomed to life as my prisoner, and clear their minds of any other fate than spending their remaining days in my confinement. At that point, I will drug them, drive them to another city's outskirts, and leave them at the side of the road. From there, it will be up to them to decide whether to find a way to reintegrate with normal society and learn how to support themselves again, or die.