IIRC, it's against federal law for a dealer, repair shop, etc. to disable a required airbag without authorization. (ex.: It's not illegal for them to enable/disable the rear door airbags on my sedan, but it would be illegal for them to disable the dash airbags without NHTSA paperwork.)
I've never seen a federal law directly prohibiting an individual from doing it to their own vehicle, although several states have such laws.
eta: If there is a fed. law prohibiting individuals from doing it, surely someone here knows the reference and will post it.