Some of my spring-planted sets have bolted too, probably due to the extremes of weather/moisture. I can confirm that blairs is correct in that you can just snip off the flower stalk and use the bolted onions first come harvest time.
I have to say that I have grown onions for many years and there are always a few that bolt so I accept it as part of the game. You have to eat some of them first, so it might as well be the bolters!!