What about name and shame? Does everyone know that this company doesn't pay? Do all of their employees know? Does their bank know? Do their customers and other vendors know? The threat of going public might help you, if you know anything about their business.
Just think about what a collection agency does, they have no special powers, they just start calling people and don't stop. It doesn't take too long to have your (outsourced?) admin go through calling everyone that works there each day.
And it can also get you sued for interference of trade.
Name & shame works only when the named party would hurt themselves more by suing you than not.
This guy has a breach of contract case (assuming his ad agreements were written properly): he needs a lawyer. Odds are he will be paid as soon as they get a letter from his attorney. None of this 2/3's stuff either: demand the full amount.
Just think about what a collection agency does, they have no special powers, they just start calling people and don't stop. It doesn't take too long to have your (outsourced?) admin go through calling everyone that works there each day.