What Rick said above, call the Humane Society, or local animal control society.
The dog is off leash, most places nowadays have rules about proper animal control.
Yes, you can protect yourself from harm, or your family from harm, from non controlled animals, but killing your neighbours dog(s) should be a last resort.
Force the problem back on your neighbours.
I have neighbours on both sides of me who have dogs, they are always in my back yard, off leash, but with their owners paying attention to their whereabouts, and they are also nice dogs, play with them all the time.