my dad (now retired) was an old school sign writer and had this sort of thing (non-paying clients) happen to him on more than one occasion.

One I remember was a shop where he spent a few days signwriting the entire front. Lots of work, most of it up on 14' tressels, and at the end the owner refused to pay. Said he didn't have to, and there was nothing my dad could do about it.

Except there was - dad went back one night with a roller, a long roller pole and a large tin of white paint, and painted the entire shopfront out. Still din't get paid, but it had two effects: one, the owner needed to get the job done again, and two: the next guy hired would be asking some awkward questions, like why did it look like the job had already been done but then painted over...