Safety equipment which is installed as a result of new regulations is mandatory. Regulations generally exempt older vehicles. But to disable or disconnect the air bag in a car which was manufactured with it, would generally be contrary to federal regulations.
Under certain circumstances an air bag on/off switch can be installed. But it sounds like you would not meet the requirements.
If you wish to pursue having it legally deactivated, check the following link. It contains more information and a link to the request form.
If you choose to simply ignore it, your insurance company could probably cancel your insurance on the grounds that it did not meet safety standards. Safercar.gov