I found this post via genvibe.com, it worked for me on my 07 Vibe:
This will turn off the 1 minute beep if the car is running and either the passenger or the driver takes off their seat belt. 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." let go of 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 for 3 seconds if you don't have a seatbelt, but the dash light will continue flashing till buckled up. It won't beep if you or your passenger unbuckles for a moment. Salvation!
To disable the seat belt alarm on a Chevy Colorado, locate the alarm fuse. Pull out the fuse to disable the alarm.
How do I disable seat belt alarm for a Ford F150 2004
Wear the seat Belt !
I find that fastening the seat belt disables the alarm immediately
The easiest way is to use the seat belt. There is also a method to described in http://forums.genvibe.com/zerothread?id=18678 for 2005-2007 models
Buckle up!
You wear your seatbelt!
take out the fuse for it
Buckle up.
By wearing your seat belt.
wear the seat bealt
You must fasten your seatbelt