Do NOT go directly to their home to collect payment. You never know what kind of situation could arise in that circumstance. I would just notify them, as Torontokids said, and put them through to a collection agency. It's totally not worth getting into a heated confrontation over. I'm not saying that YOU would do that, but they could....ya know?

I'm sorry this has happened to you. Really crappy behaviour their part - obviously NOT the cream of the crop by society's standards.