The only way to "train" your dog to be more "aggressive" is to get it into protection training. Still, you may choose not to do that and that is fine. You're better off with a lovable companion. There are leash laws in almost every state of the US and the owners can be cited for letting their dogs roam, so do contact Animal Control, numerous times, if necessary and have these animals removed by them. There is no telling when the "pack" might choose to attack someone like a child or even an adult and that is really bad news. Every dog will determine his own territory and many will "protect" that territory. Can you walk your dog the other way, away from these nasty dogs? Nag at Animal Control or the police if the pack continues to run free. People like that just make me really mad -- what's the sense of having an animal if you're not going to take care of it or neuter it or anything?????? Well, I won't go there. Good luck on getting rid of them.