yeah do NOT plug the line. not a month ago some kid did this and the resulting effect was fillup issues and then gas would push itself out of the tank, onto the side of his car from the pressure of the tank.
it takes the gastank fumes and filters out the bad particles, and he might be correct about recirculating the fumes but i forgot where the hose runs...one to the tank (duh) and the other one i forgot. maybe the intake.
i believe several people lenghthen the line and run it to the bottom of the car. if you plug it, you WILL get overflow, and my theory for this is the air in the tank is cooled by the incoming gas and then heat from the air around the tank and the road heats the air inside the tank (its not insulated like a fridge....so it will happen) and since the tank is not a symetrical shape, the air is trapped and instead of flowing out the tank filler, it forces gas out. this means you have to run below this line (about half tank?) in order not to screw your clearcoat with gas.
i would personally get a charcoal filter from a honda, i think they produce some that are MUCH smaller and you can keep those harmful fumes from making your car smell like crap. it will give you much more room for...whatever...
do a search and find that thread...its not that old and it will have tips on what canister to get....from what car and such.