Picture/s paints a thousand words, lots and lots of them, even save some samples tom post through the door of the offender if you know where they live now.
I di that to a woman who let her dog shit on our grass(garden), got zero prob if they clear it, even if they knock on the door and ask for a poo bag if they forgot thiers, I had a go at her the 4th time, then after just started smearing it around her letterbox door handle etc, with a note saying I think you forgot this property of yours, it worked.
Be a good idea to try and get some people into give quotes for tidying/clearing up the mess, extra evidence.
tabetha