The inertia switch is the closest thing to a fuel cutoff switch. Its location varies slightly depending on trim level and whether it's a coupe or convertible, but it's typically found under the dash, on the driver's side, near the floorboard. It's often a small, easily overlooked black box. It's usually resettable with a button or lever.
It's important to note that disabling the fuel supply in a 1991 Camaro requires more involved methods than simply flipping a switch. Cutting wires or using aftermarket fuel cutoff switches would be necessary and are not recommended unless performed by a qualified mechanic.
