First of all, never correct a growl. Growling is a warning that a dog gives. If you punish for giving a warning, next time he may skip straight to a bite. This is common when you see dogs biting children, the owners corrected the growl, and when the dog goes straight to biting the child next time, they say "there wasn't any warning!" because they suppressed it.
Personally, I consider human aggression of any kind to be completely unacceptable with bull breed dogs. I can understand that this dog may be very under-socialized, but it is still a danger. I think you need to contact a behaviourist to have the dog assessed ASAP, and be very careful about letting strangers into the house until then. I would put the dog away before they enter.