I had a dog poisoned by druggie neighbors years ago. When I got my new dog she was trained she could only eat in her crate. It meant following her around after baiting the house, the yard, walking down the street and verbal corrections if she tried to eat anything she found. (Leave it command). I never let anyone give her treats. Only my two older sons and I fed her.
It did work. I had them try it again and she didn't eat it. I took it to the police, but without proof as to who had put it there, they could only up patrols past my house which they were already doing. I moved as soon as I could.