I think the only way is to ask a Toyota service tech to do it.
There are instructions somewhere for disabling it in the corolla, previous gen trix, tundra etc but the same procedure does not work for the 09 matrix. At least it didn't for me after trying numerous times.
Other solutions offered up were to cut wires or clip in a seatbelt extender, or the greates reply of all is the 'click it or ticket' response which makes me rofl every time someone offers up such a smart response. not.