Well, I am no lawyer, but I am familiar with your situation.
My 1st wife was in fact a hourder. It's a sickness of sorts and as you noticed, it's annoying.
I have in fact had co workers that ran toward that tendency themselves.
I myself would start with just a talk with the power that be, I suppose cops, and just kind of suggest that they might want to pick up after themselves. I'd be gentle at first.
As they say be nice, until you can't be nice. And then continue to be nice with the help of others until you can no longer be nice.
But just a gentle little poke and reminder might be just enough to get them to sort things out a bit.
I wish you luck.
Hourding is a sickness. They just cannot help themselves. However, hopefully, with some help, they will at least clean up the yard.