Lawyer up if the app is valuable - eg has the potential to make you over 100K, if its worth less than that, send a nastygram and move on to better apps.
Talk to a lawyer even if it's worth less. Even if all you do is a nastygram, a nastygram on a lawyer's letterhead is worth far more than one from you. It won't be very expensive, but may have very valuable results all by itself.