Picking up rubish

I know that when she started out Shula had to fend for her self and whatever she found she ate. Now she has a loving home, is well fed, but still she forages and picks up rubbish. Lots of which upsets her stomach, and most worrying, people here are not above leaving poison down in places where people walk their dogs. I think I may have to resort to a muzzle which I don't want to do. I haven't found anyone who can tell me how I can overcome this, but would love to hear from anyone with a solution.