This site has some homemade remedies - the citrus one and the vinegar one both seem safe and humane to me, although I've never tried them. http://www.pet-net.net/homemade-dog-repellents.php
Last summer my neighbors lawn became the local hot pooping spot (once one does it they all want to!). He put up a small box saying please clean up after your dog and kept it filled with plastic grocery bags, which helped a lot. And I've seen him chase people down and return their dogs deposit to them very politely ("excuse me, you forgot this!") and nicely wrapped in a baggie. Of course you might not have time to watch your lawn all day!
I don't know how people can let their dogs poop on other people's lawns! The few times I've forgotten a baggie I stand there mortified hoping no one will see, then I make a big show of checking my pockets for bags in case anyone is watching and I always come right back to clean it up.