I think some people are using modeling clay, but it may well be Duco Stik-Tac (not sure how it's spelled). I suggest you try clay first, as Stik-Tac is hard to get off cleanly and out of the vent.
You can just hold your thumb or finger over that area while you top fill. This is just to stop the sponge getting too much ink when you refill by blocking off the air maze. You could probably use masking tape to do this but make sure you remove it after refilling.