OK... just a few ideas. I personally would get a male companion (or realtive - perhaps your father) involved. And the sooner the better. They don't have to actually confront or personally interact with these folks, but rather merely be visible. Have them walk with you around the neighborhood. Look around your property, occasionally step out into the street and gaze at neighbors property. Even let yourself be seen standing in the street with companion looking down to neighbor's property and even physically point towards them. Unfortuneately, society is such that a single woman, without the threat of physical support can be the target of intimidation.

Don't go overboard, but actually work on being seen pointing out your "problem" to your potential ally and protector.

I also like someone's idea of having a cop friend drive by after work (in uniform) - real slow like. I personally would not antagonize them by giving them bags of poop or calling the landlord (unless you are absolutely 100% sure the landlord is "going to evict" them). This will only cause you more problems. Yes... I think you want to try the head game first. Why not - you're a woman and my wife attests to the fact that she (and other women) can be quite good at this. The silent ominous threat of a potential confrontation with your protector could be just enough to keep your problem at bay.