I think you should display a message like Panic did before with Transmit when people entered pirated serials.. Don't reprimand but be polite asking them not to pirate your stuff.. The effect you're looking for is for the pirate to feel a bit guilty...

You can then either limit the functionality to a free lite version of the app, or put a time limit for them to try or just disable the app...

If you plan on putting a time limit, it might make better sense the show that message after maybe 2 or 3 use of the application so that the user has already tested it and likes it.