my dog did the same!!
unfortunately, I think it's a bad habit that, once they've got, the only solution is to clear up before they can get to it. Sorry!
.. of course, I did this religiously, and my dog discovered cat poo - even worse!!!
EDIT: She *has* been much better since I moved house - and lost my job - so she now gets 2 or 3 long walks a day instead of 2 shorter ones, and company pretty much 24/7, so i wonder if it was part boredom..?
EDIT 2: Doh! of course, as a result of the walks and me being there, there's been less (none?) for her to find, too.