I'm glad to say I don't have any experience with this problem, but assuming we are talking about a masonry chimney, there is nothing you can do to stop the movement. Once started, the movement is likely to continue and possibly accelerate. It may be possible to arrest the settling at the foundation by injecting hydraulic cement or evaluating a soil or saturation problem. You need to call a specialist before this gets out of hand and repairs are infeasible. I would start with a reputable foundation contractor in your area.