We told him when he was three that he could only have a nuk in his bed. (He was in a big boy bed by then.) If he was in need of a little soothing, he'd go to his room, sit on his bed, and use his nuk for a few minutes, then put it back in the bowl on his bedside table, and resume playing. He used his nuk at night, still.
We told him that when he was four, the nuk fairy would take his nuks to a baby who needed them, and bring him a cool toy instead. We took them away one night and never returned them.
He ended up finding a new spot to go to when he was in need of soothing -- the bottom step of the back stairs, next to the shelf with library books. He sat there and read books when he needed to settle down.