Get over it. This happens everyday a thousand times over. Provide a better service that keeps users from going elsewhere. Keep innovating to keep the competition one step behind.

Heard of Pinspire? The Samwer brothers have used the copy game over and over again to get mad rich.

It's not worth your time getting upset. Look at it as a form of flattery and move on. Your time is better spent on making your site better than bitching and/or attempting to get them to stop.