I know this thread is a bit old now. But for those still looking to stop the seat belt chime the easy way.
Simply unplug the connecter at the rear of the drivers seat (underneath of course). If you raise your seat and access the rear under part of the seat you will see a white plug/connector with yellow tape on it. The plug has 2 wires (red and black). Simply unplug this and it will stop the seat belt chime.
This plug is not connected to the side air-bag in the seat. all air-bag connectors are Yellow for that reason. The side air-bag connector is towards the front of the seat (driver and passenger) on the outer edge tucked away behind the side plastic cover.
Cheers,
Phil