Collect a lot of it. Follow them home. Leave it on their doorstep with a note. Maybe "I've seen you left behind a lot of poop so I thought I'd return it to you. If I find any more I be certain to enlist my neighbors to help bring you more."
Earlier this year I saw a lady allowing her male dog to lift his leg right on one of my newly planted lilies. Yikes! I ran out to the curb, got in her face, and screamed "BAD OWNER! BAD! BAD! BAD!" She put her tail between her legs and skulked on, dog in tow.
I honestly like all dogs. People not so much sometimes.