Just leave them an anonymous note. Be polite. Sometimes people are just oblivious, they are not always trying to be rude. Let them know that it is bothering you. And ask if they wouldn't mind picking up. Even leave them some doggie bags.
Is there some sort of litter/pet waste law where you live? Read up on it and if they're still not picking up after their dog, take proper action according to said law. Hope this helps! :)
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