Obviously a cork screw is the way to go if it's long enough, but if you need something longer I'm sure the hardware store has a long coarse threaded screw (something designed to screw into wood) that you can use like a cork screw. If it's loose enough that it rotates, try to wedge a spoke (or something thin) in between the cork and the handlbar wall. If you can get enough spokes in there, and move them around so they aren't all in the same place, then pull them all out at the same time at the same time, that might bring the cork with it.