Esa Paulasto is correct that at this point, taking it up with your local government department that handles this is probably the next step to take.
You could look into the laws in your town, I know most towns I've lived in had laws where it's required for people to clean up after their pets. Leash laws would also come into effect here if your town has those. For either of these, you can call your local police on their non-emergency number, and they'll probably send out an animal control officer to check it out.
As far as deterrents go, most deterrents that will work are also bad for your lawn. Mothballs and ammonia are two that I can think of. You could try a motion activated lawn sprinkler, but some dogs like to play in sprinklers, so it might not have the desired effect. Springer Spaniels are hunting dogs, and I would be willing to bet that they'll get over any initial shock over the sprinkler pretty quickly.
I've heard ground pepper suggested before, so if you can buy ground pepper in bulk, you could sprinkle that around. The idea is that it irritates the dog as they're sniffing for a spot, so they look somewhere else. Ashes, especially tobacco ashes, should work in the same effect too. I don't know if either would have any effect on the grass. I'd think they would also wash away in rain pretty quickly.
If you aren't too worried about your relationship between you and your neighbor, an effective way to get the message across, and to get them to at least make sure their dogs don't go on your lawn, is to send the dog's poop back to them.
- Put it in a bag, and attach it to the dog's collar so the dog brings it back with them. (Only works if you can catch the dog)
- Put it in a bag or a box, and leave it on their doorstep. (Passive-aggressive bonus if you leave a note saying they forgot it, extra bonus if you don't use a bag or a box.)
- Straight up, just toss the waste back into their yard so they can deal with it like they're supposed to.
I'd start with the government officials and deterrents first though, as it's best to avoid wars with your neighbors.