I'm informed by a spokesperson for Toronto Parks that By-Law Officers are similarly unimpressed by irresponsible dog owners, who not only think of our "greenspaces" (that's the way it was put) as dog toilets, but especially when , just as the owner passes the by-law sign, unhooks their canine(s), because, um, "Dogs have to be free." Some parks e.g. in the former (? ... Amalgamation will never really happen in peoples' minds) North York, very old signs still stand, stating that it their parks are not to be fouled (I don't recall the exact wording -- anyone?) by dogs. Enlightened.
If you have a dog (or dogs; I won't get into professional "dog-walkers" with six or eight dogs dragging them through the area; should there be a limit?) and you have a lawn, that's where the end-product of the animals' digestive processes should take place. It's just not good enough (though better than nada) to perfunctorily pick up a bit when it happens in a park; you rarely get everything anyway. If you have to use parks, please be scrupulous about removing the fecal material (As the majority of owners, I've noticed, are). When I see small children rolling about on the turf in a heavily-used park, I cringe. Punch Toxocara canis into that Wikipedia thing for a little bit on canine-borne parasites. Even without the zoonotic nasty bits, I don't like to see anyone sitting in excrement, even if it's just a little bit of excrement. But if the dog's owner complies with the by-law, then she/he is taking responsibility. Just makes me a bit queasy, is all.
Two suggestions: If you can capture an image, that might help. Maybe a "Facebook" page ... but say something, provided you feel safe and there are other folks around. No need to be rude, but social pressure is very effective indeed. When was the last time you saw someone light a cigarette on, say, the subway? (When the smoking bans started -- 30 years was it? -- there was an adjustment period, and people smoked in shopping malls, at concerts and films, and so on. It was all so new, and seemed so normal to have a smoke at a bar, that people (in general) said nothing. Think about what would happen if someone lit up in, oh, a library (I remember ashtrays in doctors' offices!) now ... I don't know what that would cost you, but it's probably more than the cigarette.
The other necessary approach: Call the city's by-law branch and complain, every single time you witness someone leaving their pet's little (not always that little) gift behind. If it's the same place, apparently by-law personnel will pay regular visits to that park. Note the time of day if it's relatively consistent. Every single time.
I'll post the number to call when I find it.