Ask HN: Copyright Infringement

About a year ago I did some contract work for a well funded startup (NY based). I was commissioned to build out an android application from their current iOS port. When we were about 90% finish the client decided that some of the issues and remaining bugs were not being addressed in an orderly fashion. We requested that the developer that built the iOS part, filters in this instance, would provide an helping hand in trouble shooting the problem, but they didn't want to do that as well, so I gave the client other options for addressing this issues, they din't like them as well. So at the end we ended the contract since they did pay a deposit (25%). Now today I did a search on the playstore for some odd reason and found that they have published their app. Curious to see if they just repackaged the sources that I developed or rebuilt it all I decided to check out the apk. To my shock they still had all of our sources,package names and all. Yes some code has been removed/updated but the bulk of the app still uses code that was developed by myself. I sent off a Cease and Desist Letter , but I highly doubt that would do anything giving how my last conversation went with the client. So I was wondering if anyone has ran into this problem before and what have they done to either have the product taken down or receive the remainder payment. And yes I have a contract in this case .. the unpaid amount is around 12k. Thanks