Got this great tip off Stargazer's STI blog...it works
One of the "features" on the 2006 Imprezas is that if you don't buckle your seatbelt within 30 seconds, it turns on an incessant beep. Now, I never drive without my seatbelt on. I feel naked without it. But there are the occasional times where I'll be in my garage or parked somewhere working in/on the car and that beep is very annoying. The owner's manual does explain how to temporarily disable it, but it reverts back to normal the next time you start the car. Here's how to disable it more permanently:
1. Turn the ignition to ON. The engine should be off.
2. Within 30 seconds, insert the buckle in and out 20 times. You can hold the release button down if you like. Just make sure to remove the buckle all the way so that it "registers" in and out.
3. The next time you start the engine, you'll get the initial seatbelt chime, but it won't give you the continual warnings afterwards. You should notice right away that after the initial seatbelt chime, the seatbelt light will turn off and no longer flash.
I had to try it a couple of times until I got it right, but it works. To reset it back to normal, just disconnect the negative connector on the battery for 30 seconds and reattach.