Can do.
Turn the key in the ignition to "on", the spot just before starting the engine. Buckle and unbuckle the seatbelt 20 times within 30 seconds. Immediately take the key out of the ignition. Enjoy your chime-less Subaru
Edit: you only have to do this once and the chime is off for good. Unless your battery dies. Also, you can toggle the chime on/off by following the same instructions.