First, let me say I am so sorry you have gone through this. As a former pittie mom (our 14 yo boy crossed the rainbow bridge at the end of last year), here are a few things that most don't realize about owning these dogs. THEY ARE SMART. They CAN have above-average prey drive. They love chasing anything that runs and can get into serious trouble with neighbors for various reasons if not adequately contained. We NEVER let ours loose without a leash, and if he did slip the door, it was all hands on deck to round him up, not because of a danger to humans but to other animals. We knew this and did everything we could to mitigate the possibility he would get loose. Regardless of your feelings toward the breed, it is the humans at fault for not properly containing the dog and/or knowing where he/she is at ALL times. Once a dog of any breed has shown that they tend to go after other animals, the owners MUST begin working with the dog to break this habit; if they feel they can't do this, they need to review their containment options and fix them. Ultimately, the owners are at fault. As to ways to keep the dog out, I don't have suggestions other than the ones previous posters have suggested. Electric fences would be a great deterrent.