Domestic long distance or international long distance?
I don't think there's a way to get Domestic Long Distance blocked.... simply because it's included in your minutes. If you're in Atlanta, you're charged the same amount monthly whether you call Atlanta, Duluth (still in Georgia), Boston, San Francisco, Seattle, or Juno. You can get Family Allowances that puts limits on numbers that can be called / received, but it's a list of numbers. You can't say "yeah, little Johnny can call anyone with a 404 area code, but not an 808 area code."
If you're looking for International Long Distance to be blocked (to keep people from calling like Canada, Mexico, Peru, India, etc), you can contact Customer Care to have that [free] block placed on.