If I see it happening, I go out with some doggie bags in hand and very politely let them know that if they ever need a bag to feel free to knock. If we aren't home there will be some left in the mailbox too. It's all too easy to take off on a walk and forgot a bag. It's a very friendly gesture but it also makes it pretty clear you don't want their dog's crap in your yard, or anyone else's yard for that matter.
+1 for that. It would probably get the point across without making your fellow neighbors mad at you.