You might be able to use DMCA to do a takedown, but all that would do is shut off their website for the time being. The code could still be shopped around (illegally, of course).
You might be able to interest the state or feds in prosecuting for theft and copyright violations, but that would be a matter of luck, I think.
Best thing would be to contact an attorney to see about filing a civil suit, but that would cost serious money. It depends on how important the code is to you.
There may be other options; these are an offhand list of what comes to mind immediately.