As far as I remember, it is not possible to make an inverse from a trigger-event, i.e. the OnSomething-Events with the tiny green arrows next to them. This is a restriction that kind of makes sense, since triggers do not describe a state (like IsOverlapping), but rather a "once-in-a-moment"-event that can hardly be inversed ;o)
However, BluePhaze's approach looks pretty solid! Hope it works for your game!!! ;-)
Regards,
Chris