Use a solid push in rubber plug/remove the flapper and linkage arm if desired...

Why bother?

...

So every non CA car caught on fire?

Come on Mike

Remember to never carry a fire extinguisher, or 2.

All three carbs have a bowl vent in the standard location, cast into the rear of the air horns. This provides atmospheric pressure in the bowl and that's why the engine runs fine regardless of whether the center carb has a working second vent.

The vent on the center carb, if present, should only be open when the throttle is at the curb idle position. Curb idle for an emmisions carb is fully shut; that is key off, so throttle-stop solenoid is retracted. In other words, the external vent is closed when the engine is running.

See here: pages 32.
http://www.hamtramck-historical.com/ServiceHighlights/1970PassengerCarHighlights09.shtml

Whether there is enough vapor that could exit the vent and start a fire does seem rather dubious. Every vehicle until the vapor savers came along have open vents. The only difference being that most were protected by the air cleaner. Doing so keeps dirt out as there is air flow into the bowl as fuel flows out and fuel level drops. This is not to say that once there is fire, fuel vapor present would not contribute to its growth - of course it will!

That car had a problem. It was going to catch on fire weather that vent was plugged or open. If the carb is going to overflow fuel it will come out onto the external parts of the carb and Intake manifold,

How do you know if it had either vent style or no vent at all? What was the cause of the fire?

Now these carbs are like Pinto gas tanks? They going to ignite, and soon. OOGA BOOGA!!