Two things come to mind. Are the neighbors giving the pup treats and/or lots of attention? It may be exciting for your dog to go there. Secondly, have you ever worked on the boundaries? Walking the dog on a leash around the perimeter rewarding when staying in, correcting when straying? Just my 2 cents worth.
Hail Gunther 7-2-2013
If at first you don't succeed, destroy all evidence that you ever tried.