The pilot landed after performing a local flight with a passenger. The passenger, who was seated in the front seat, exited the airplane. The pilot then departed on another local flight. During the approach for landing, while turning from base leg to final, he was unable to move the left rudder pedal. The airplane entered a right hand spiral and impacted a tree and terrain. The airplane sustained substantial damage to the fuselage and wings. A postaccident examination of the airplane revealed that front seat belt buckle had jammed between the rear seat rudder pedal and floor board.