Talk to the neighbor with the dog, other neighbors about the dog (just to see if it's all bark and no bite), and check into your local laws/regulations.
Sometimes, the local regs will be on your side, more than the owner's.
One of my brothers lived in a neighborhood where one of the residents let his 3 aggressive and territorial dogs roam all over the place, and refused to listen to complaints. The town LEOs claimed they couldn't do anything about it.
When the dogs knocked his little girl off her bike and looked like they were about to attack her... he kicked the crap out of them, until they ran off. (He would have preferred a weapon, but didn't have time to grab one.)
When they came back the next day, and were even more aggressive and threatening, he put one down with a shotgun. Police responded to "shots fired" and "dead dog in the road" calls.
When the responding officers couldn't find any witnesses, one of them told my brother, "I know you did it, and that all these people are covering your [butt], because they hated those dogs too. But, they really don't need to, since everybody knows those dogs are aggressive; and it's just a $25 fine for discharging a firearm in the city." (-just an infraction, not even a misdemeanor.)
To which, my brother retrieved $50 from his wallet, and replied, "Can I prepay for 2 more?"
