There's no way to stop it that I know of. What you can do is vigilant monitoring and rapid response, as speedshopping wrote. Use the DMCA as needed. It's a lot like protecting a trademark. If you don't get proactive, you can lose the game.
I added this thread to another thread but seems more appropriate here - can anyone help?
Can i ask if anyone has any experience with our potential situation: about 8 wks ago i found a .info domain that had identically copied our .co.uk website and google had managed to index 3,000 pages before i got the content removed - however, could this be the reason for our recent drop in rankings for over 100,000 keywords - the rogue domain still has 350 pages in google (viewing these pages just shows empty html), however when i look at googles cached version it states the page is our .co.uk page and not the .info one!?