Well, his/her handle does say he/she owns a "Toyota Camry Hybrid"!

Seriously, though... while we're waiting for Jesse to provide more clues [or step-by-step isntructions], here's something else to try (in desperation?). Based off of the same Matrix board, I read the following from a certain "Bazirker":

I got mine to turn off by adapting instructions on how to get the Prius to stop beeping in reverse. Here's how I did it:

Get in the car and turn the key to on (don't start the car,) then change the odometer to display "odo" and not trip A or B. Turn the key off. Turn the key back to on. Within six seconds of doing this, press and hold the odometer button. Do so for 10 seconds. Then, while still holding the button, buckle up, edit: then unbuckle. The "odo" display should then change to say "b-on." Push the odo button to get it to read "b-off." Then wait for the display to change back to the odometer. That's it, you're done. The car will still beep right when you turn it on if you don't have a seatbelt, but it won't beep if you or your passenger unbuckles for a moment. Salvation! If you're having trouble getting this to work, I did this on my 2005 Matrix XR 5-speed manual with the door closed and no one in the passanger seat.

[...]

Yeah I think you have to buckle then unbuckle the seatbelt, which I originally didn't type. I editted it so that's in there now. My bad

Do you feel like ruining yet another couple of fingers and trying it again, this time WITH unbuckling as well? I know I didn't try THAT combination as yet...

===> EDITED on Dec-07-2006: FL07THC tried it as well, same result... Don't bother trying yourself