Whilst being a dog owner myself, the idea of putting poisoned food down horrifies me. Please, please, please don't do it. I know how annoying dog fouling is, but no matter how bad it is, putting the dog through pain and the owner through distress is not the way to go.
When I walk my dog, it amazes me the amount of 'mess' that is left behind. I have resorted to following another dog walker whilst walking mine to see if that person was the offender, but even in that case, being able to challenge them after the incident would not result in the solution being solved but no less a confrontation of sorts.
If the problem proves to be too much, then I am afraid the solution is that you will have to catch them on CCTV, and then submit that to the council for prosecution.
Owners that do not pick up, aren't doing it because they don't know its wrong, there are enough signs around the streets to tell you its wrong! They are doing it because they are lazy and are bad owners and no doubt would not give 'two bob' if you told them to pick up.