I work for a Veterinarian and answer this question easily 20 times daily: Neutering the cat won’t stop the spraying- oncea cat has “learned” to spray, it’s a hard habit to break… I do agree with keeping catsinside if at all possible- both to keep from annoying neighbors and for the health and longivity of the cat’s life…inside cats tend to live much longer than their outside counterparts. JMHO Lisa G.
In the mean time, talk with the neighbor, make them aware of the problem, and ask that they contain the cat or get it fixed to avoid this problem.