If you are squeamish about smushing it and it lands on a flattish surface, you can put a glass over it and slide a piece of card stock or cardboard between the surface and the glass and then carry them outside pressed together so that the hornet remains trapped in the glass with the card covering the open end (you must hold the card and the glass together or the hornet will escape), at which point you can either release it or leave it to die under the glass.