There isn't really anything you can do, other than refuse to see him again without transferring the lost earnings, or insisting in fixed pay with the lost pay on the next meet.
I would just say in the future, get all the potential earning up front, or pause to get paid if they want to extend. Don't continue unless you have cash in your hand.
There is nothing earned by using what you know about him other than to post a warning (like you have done), as the police won't take any notice. But if you haven't you should consider also reporting him to NUM (national ugly mugs), which has a pretty broad reach. Anything like outing could lead to a lot of problems for you. Best to keep it discrete.