Charcoal Canister Vent Valve - How to Close

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My code reader indicates a gross vapor leak. I replaced the fuel cap and reset the error (the error repeated). I used a vapor system smoke test tool and found a large leak at the Vent Valve but this valve is Normally Open. How can it be closed so that I can test the rest of the vapor lines? I plugged the purge valve outlet as I didn't know how to close that valve either.