i have found when dealing with problem pets that it is generally the owners that need retraining
not many dogs can't be retrained, it requires someone with specialised knowledge and time to achieve though
but in todays world this takes time which equals money
once retrained it will require someone else equally knowledgeable to look after it
so the easiest answer i am sorry to say is to put the animal down
law should be that people should have to sit a test before they can own a dog to prove they are capable of bringing up a well adjusted animal
mind you though people sit drivers licenses to prove they can drive, some are more dangerous than a bad dog