Originally Posted by Kit Fox
The passengers side airbag? Open up your passengers side (not sure on LHD cars) and insert your key into the slot that says passenger air bag and select off or on.
EdiT: NM, didn't read it through.
Injure or kill your passenger and you will have more than a broken windshield to worry about. Your insurance company may not want to pay for someone killed or maimed because you turn off the air bag. These are to be turned of for child seats not on a whim.