Once a bed is infected with fungus, it'll stay there, as long as tomatoes are planted there. If that bed is your only option for tomatoes, you'll need to use a stronger fungicide such as chlorothalonil, or even a systemic, as a preventative (as in, before the blight starts). If you can rotate your beds for a year or two, that will help. Once the blight sets in, there aren't too many options.

Do you get a crop at least? If you don't get a crop, you could think about growing in buckets or Earthboxes. In the meantime, you could solarize the soil in the veggie bed and/or grow a green crop there.