Did you really figure it out? You can also make the event with no graphic so you can walk through it. When you change the opacity, you will still run into it unless it's a tile set.
Also, if you used a graphic at first, then wanted it to disappear, then you could just flip a switch, or a self switch after the event is over. Then make a new tab and for the next tab set your condition as that switch, then with no graphic. That way the event can still be used to talk and things like that, but it won't be in the way visibly, or physically.