Here is how to solve the problem (for a 99 civic, DX)
The solenoid valve (Vent shut valve) is located on the charcole canister against the engine fire wall (left side, passenger side).
Remove it from the engine cavity.
Remove the valve from the canister. We had to heat up the two screws and melt the plastic a little to pull the plugs out. There was no risk of breaking the charcole canister.
Test the device: Measure the resistance of the device. Should be around 25-30 Ohmes. If it passes, then put 12Vdc on the two prongs and see if the solenoid functions. You should hear a solid "click". I blew in it and tried to see if I could resist the flow of air. It woulnd't close, so I knew it was jammed.
Using pliers, bend out the four metal tabs and start disassembling the valve. Take it right appart until you have access to the white outter coil and the inside part. Using a dremil, sand the inside of the white coil a little, allowing a little gap to move the solenoid core. REASSEMBLE.
This solved my problem 100%. Note: When filling my gas tank, before this fix, I couldn't fill at full max. Gas would fill the tub and click off. With the repair, now the gas fumes are collected into the charcole filter, and I can fill the tank at full speed.
Note: The dealer quoted 400$ to fix this problem. To repair this part, it required 1 hour of work. Totally doable by anyone mechanically inclined. Good luck!