I would have done exactly as you did, except that I would have said a number of choice words to the guy for allowing his dog to leave his yard. Neighbor or not, if his dog is aggressive it, he should already know that and take extra precautions (such as a fence) to make sure his dog can't run free.
Then I would take a paper bag, fill it with dog poop, put it on his front step, and light it on fire.
