I have this problem with neighbour's dogs too (to be honest it is the least of my problems with the neighbours though!) I think speaking to them is the best starting point, and yes, mention the toxic substances you regularly spread for pest control or weed control etc. that way you can make it as neighbourly as possible, not mention the pooh at all, and stay on good terms which is always the best option.
JohnJay, I don't think advocating animal abuse like kicking dogs is at all in order. It is not the dogs fault, he doesn't recognise human fences and is just being a dog. It is his owner's who are to blame, and abusing the dog is not helping anyone. You are also mistaken as regards the legality of a dog having to be on a lead when off the owner's property. The control of dogs act states that a dog should be under control, not on a lead. There is no obligation in this country to keep a dog on a lead unless it is of a breed or crossbreed from the restricted breeds list.
This dog, and no dog, should be allowed to wander, there I am sure we all agree. It is illegal to allow a dog to wander unsupervised and not under control. It is absolutely legal to walk a dog without a lead, so long as you pick up the pooh and have the dog under control. I have four and 2 need leads to control them. The other two do not.