Recently, I've had the pleasure of dealing with neighbors who think that their backyard is a great place to let dog feces do what it does best: bake in the sun, melt in the rain, and basically smell up the whole joint. My bike gets parked at a prime spot inside the radius of poop smell, and it's been fantastic thinking about how this summer is going to be when we try to barbecue while the smell gets worse because we weren't proactive about it.
I find a lot of articles online about ways to deal with neighbors. They all say the same things in the end, though literally zero of them are about what to do when your neighbor won't pick up their own dog's shit in their own yard (what are we—in Texas?), but this is how I've been dealing with it.
Talk to your neighbors. In my case, I tried to talk to them a few times, but I am a sissy and the opportunity never presented itself. Eventually I left a (nice) note. No threats, no nastiness, just what I would have said face to face if we were ever outside at the same time: the poop smells pretty bad, and it's been getting worse lately. We want to enjoy our backyard, since it's one of the reasons we moved into this place, and we understand that it can be difficult to be on top of this, but we'd really appreciate it. Simple and to the point (not passive aggressive) and not anonymous; I signed my name and let them know which unit I was in.
Report it to the city (or HOA, if that applies). No one can be less surprised than I that my note tactic didn't work, and I didn't want to leave any more because as nice as it was, more notes could be considered harassment. Once I left the note, I realized that it was silly and pointless, since now I couldn't really talk to them about it. Besides, I pay taxes so that the local government bodies responsible can take care of crap like this for me (pun intended). Since a government official has gone over there, the problem has diminished greatly, and by that I mean I haven't seen a single poop stay on the premises for more than two days, and I call that a win, even if my neighbors don't like me (I could have gone over with wine/brownies/flowers, but I'm not that person. I don't like them, either, but I'd rather be the person calling the city than the person letting feces fester in my (and my pet's) environment). The only important thing I learned by doing this step was the next step:
Call Animal Control. We aren't there yet. Are there still steaming piles of poop out there occasionally? Sure. Do the neighbors know I am the one who sicced the City of Somerville on them? Unfortunately, I'm positive they do. On the other hand, I have this tidbit in my back pocket—animal control is a separate entity from who I contacted through the city's website (Somerville in my case, but you can always check your local ordinances), and they can issue fines. This could be your first step if you really hate your neighbors.
I meditate. I try practicing lovingkindness...but at the end of the day, I am literally dealing with shit here. City anonymity is so frustrating that it takes it to a whole new level when your neighbors who are less than thirty feet away from you are doing things they wouldn't do under a microscope. And that's why these local government offices are in place, right?