Copied and pasted from this thread:
To remove seatbelt chime
1. Turn the ignition switch to the OFF position, and fasten the drivers seatbelt.
2. Start engine
, and wait for Seat Belt warning light to turn OFF.
3. Within 60 seconds of starting engine, unbuckle and then re-buckle driver's seat belt at least 3 times, ending with belt buckled.
4. Turn off the engine. A single chime will sound to signify that your life is in your own hands.
It can take several tries to get it disabled.
It works. The seat belt warning light will remain on.