Agreed under the Jail!
A box. Just big enough to stand up in and lie down in. One slot, for food. One hole, for waste. Ventilation and bright illumination, 24 hours per day. No clock, no entertainment, no sound, no human contact whatsoever. One wall completely covered with a picture of his daughter, looking at him, behind plexiglass. Keep him in there until he goes insane with guilt and commits suicide by bashing in his own skull, or starves himself, or dies of some disease. Then just fill up the box with concrete.