I was wondering the same thing a while back. This is what i've learnt from Geelak900 and some posts on Nasioc but i haven't actually tried it:
"Reach up under the steering wheel and unplug the "integrated module" - it's a small black box. pry open the box and put a piece of electric tape over the piezo speaker. or rip out the speaker if you never want to hear it again. with the tape, it's much quieter and still functional, yet removable.
In order to disable it "permanent'y" (until the battery is disconnected), do this all within 30 seconds:
Open and close the drivers side door. Turn the key to the on position. Buckle and unbuckle the seatbelt 20 times within 30 seconds. Remove the key and open the door. Then, start the car. The seatbelt chime will go off 5 times (every time it is started) and then no more while driving.
HINTS: Never fully take the seatbelt out of the buckle. Release it and put it right back in. Also, make sure you count when the seat belt is released, not when you put it in."
-STI_Crazy