Bad Mouthing a Ex-Client

I have been doing business for 7 years with a client and we have gone our separate ways after both parties having tremendous gains. However, they feel they do not have to go thru with the final payment that they owe me. This group has collected a lot of money due to the success story that I have brought for them and I would like to take their inside story of how they have lost investors money due to bad business decisions based on wrongful events.
Basically i want to verbally tell my story back it up with proof and have that all written out.