I say this as a Pit Bull owner for the better part of 13 years--kill the owners!
Pit Bulls have received their reputation not because of a genetic flaw that alters their brain chemistry, makes them inherently more agressive, or in any way vicious. It is the dog owners, not the dogs, who are bad.
Dogs, like people, can have physical conditions that alter their patterns of behavior (sometimes for the worse), but more often than not, it is a byproduct of poor training/ownership. Much in the way parents are held responsible for the actions of their children, so too are dog owners responsible for their animals.
Involve the police, not just animal control. Direct them to the home of the owners...accompany them if need be. If they have a car illegally parked in their yard, call the police. If you witness them jay-walking, call teh police. If they are playing their stereo too loud...call the police. A bad dog owner needs to be punished, if not for having abused their dogs (which these dog owners clearly have) but for anything else you can bring them down for. Remember, Al Capone went down for tax evasion.
Just as you would not feel good about sitting idly by ignoring clear evidence of child abuse, so too should you not ignore this abuse.