1. Go talk to them and ask what can be done to solve the problem
2. If that doesn't work, I'd talk to them again and let them know you'll bag the poop and set it back in their yard.
3. If that doesn't work, you will probably have to build a fence. Most people who don't bag their dog's poop and let them run into other people's yards are not going to consistently do what's necessary to solve the problem.

You can always lodge a complaint, but then it will come down to "That's not my dog's poop, must be somebody else's" kind of argument.