Usually when one curls into a ball it is injured or close to death. Believe me....if it was unhurt it would be flying around looking for a way out or for someone to sting.....hornets have very bad attitudes.....lol. Are you sure it's a hornet or a bumblebee.....they do look kinda sorta the same. If it is not flying then more than likely it is injured and should be simple to kill. Take a broom and sweep it to the floor and then the rest is easy.
I think if he is curled in a ball, he very well may be Swimmin with the fishes? I don't think They are smart enough to play "Possum". But I was chased by a porcupine that did know this trick! Now give him a little extra blast of OFF- then you do not have to worry. Or the Lysol treatment, but I can't describe this because it is a practiced art.