May I suggest that amonia may not be the best thing, as urine develops amonia as it sits and so the two smell somewhat alike. Nature's Miracle that you get at a bit pet store is very good and apparently masks the smell of the urine to the dogs, at least to a pretty good degree.
Neutered or not, no dog should be getting away with this, as it is probably a leadership issue. As much as I'm all for motivational/reward training methods, this is a behavior which I WILL tell the dog, "NO!!!!" And take him right outside. He is claiming ownership of your stuff and thinking he's all that. So, you have to let him know that it is YOUR stuff and that YOU have everything under control. He does not need to "carve his initials in the tree." What are your interactions like with this dog? What kind of training does he get? You may want to beef up the obedience training and impliment NILIF more to make sure he understands that it is YOU who control his resources and that you are taking care of things....he doesn't have to.
http://www.dogbreedinfo.com/nothingfree.htm
You'll need to really watch him every second for a while....really try to prevent even one more marking episode.
Good luck.
