I'm thinking the scene in Me Myself and Irene where Jim Carey finally cracks and takes a poo in the neighbours toilet. Classic!
Some posters here sound like the kind of people I'm so happy that they dont live in my neighbourhood. Suggesting putting it thru the letterbox, or back in his yard, report them as missing etc. is only going to escalate things. You have to live alongside these people so you need an amicable compromise if at all possible.
Here's what I would do. I would approach the neighbour again having rehearsed exactly what I intend to say. The following points need to be made: you are very unhappy that dogs keep pooing in your garden; there are no other dogs in the neighbourhood so it seems likely it is his dogs; if he wont accept that it could be/is his dogs then you'll be forced to get a photo of them doing it which seems a bit petty; ask him what he will do if you get photo evidence. I would also try and deomnstrate to them that you are reasonably by saying that you're not a dog lover, but that you accept that they'll bark when let out in the morning at 7am or whatever. There's nothing you can do about it, so dont make it part of the issue. Besides, it is the only thing you can do when you have dogs.
If the neighbour refuses to budge, then get the photo. Show it to him. At that stage you tell him that you'll have to report it to the autorities if he does not stop it happening. Try to keep calm, and display that you are reasonable, that you're only doing this because you are really upset that it continues to happen, and that you have no alternative but to adopt this approach unless he does something about it.