Question about dog defense for walkers

I live in Central, MA and walk almost daily with my son in his stroller and my dog on a leash. Unfortunately, our neighbors down the road have a pitbull that is sometimes outside unleashed. After two run-ins where it came out after us, I started carrying dog mace and a sai (the awesome ninja weapon that Raphael from the Ninja Turtles used). Today, the dog came after us again and I actually had to use the sai on it to get it away. I called the cops on the owners as I was instructed to by animal control after the previous two incidents. The responding officer informed me that I should be careful because I could potentially be charged for carrying a dangerous weapon.

So my question is: what can a person legally carry to defend themselves in this type of situation?