Maybe try something like: http://olddebts.com/

You have the date of the last invoice no? With something like Old Debts it will at least hit their credit rating if they don't pay up. I recently had success with this and got money out of a client.

That unpaid balance will follow them for a long time and they will have to resolve the issue with you.

Beyond getting your money back I wouldnt take revenge by going negative publicly and creating drama.